Registering a Used Phone with an au SIM Card in Australia
Buying a second-hand smartphone in Australia can feel like a bargain right up until you insert a new SIM card and the phone refuses to cooperate. Carrier locks, activation locks, mismatched network bands and forgotten account PINs are all common stumbling blocks, and none of them are obvious when you first pick the device up. Getting a used handset working with an au SIM is a straightforward process once you understand the sequence, but the order of the steps matters enormously.
The good news is that au runs on 4G LTE and now 5G networks, and a large share of the second-hand market already contains au-compatible devices. The trick lies in checking the phone's status before you commit, unlocking it properly if needed, and confirming that the network bands inside the handset match what au expects in your area. Get those three things right and the rest is largely administrative.
This guide walks through the whole process, from the checks you should perform before purchase to the moment your new au SIM connects and starts registering to the network. It also covers the specific issues Australian buyers encounter, including the differences between Japanese carrier-locked models and open-market handsets sold locally.
Before You Buy: Checking the Phone's Compatibility
Start with the physical SIM tray. Japanese-market handsets frequently ship with a nano-SIM or a combination tray that also accepts micro-SIM cards, whereas Australian au devices typically use a standard nano-SIM. If your used phone only has a micro-SIM slot, a physical au nano-SIM will not physically drop in without an adapter, and forcing it risks damaging the tray. Confirm the exact SIM size before you hand over money.
Next, check the network bands. au operates LTE Band 1 (2100 MHz), Band 3 (1800 MHz), Band 11 (700 MHz), Band 18 (860 MHz), Band 28 (700 MHz APT) and Band 41 (2500 MHz). Most modern devices sold in Australia support these bands. The Band 11 and Band 28 coverage matters particularly in regional areas, and Band 41 is what carries the bulk of au's 5G traffic in cities such as Sydney, Melbourne and Brisbane. A Japanese domestic model may be missing several of these, so verify the specification list rather than assuming.
Beyond bands, check the carrier lock status. Any Australian second-hand seller should be able to show you that the device is factory-unlocked or carrier-unlocked. If a phone has been locked to SoftBank, docomo or Y!mobile — the three Japanese networks alongside au — and is now being sold in Australia without being unlocked, it will reject an au SIM outright. Ask for proof of unlocking, or buy from a reputable reseller who offers a statutory guarantee.
Handling Activation Locks and Network Locks
Even an unlocked phone can carry an activation lock. On iPhones, this is the Activation Lock tied to an Apple ID and Find My iPhone; on Android devices it usually appears as a Google account lock or a Samsung account screen demanding a sign-in. A device still linked to a previous owner's cloud account is effectively unusable until that account is removed, and a seller who cannot complete the removal step should be avoided.
The Network Unlock Code (NUK) is a separate issue. Japanese carriers including au and docomo place a SIM/network lock on devices sold on instalment plans, and this lock can persist for a set number of months after payment completes. Some Japanese importers and online marketplaces provide the unlock code, sometimes with instructions for the au Web account, but others do not. If the handset came from a Japanese auction or a local Japanese marketplace listing, treat the unlock as unresolved until you have evidence otherwise.
There is also the SIM lock on the iPhone side of things, where an Australian carrier-locked handset from a previous owner may still be restricted to that original carrier. Any network that has supported "carrier unlocked" devices for a reasonable period can request a removal, though this adds time to your purchase. Again, an Australian seller who has already handled these steps will have the paperwork ready.
The au SIM Registration Steps
Once the handset is confirmed clean, the au SIM registration process itself is quite simple. au uses nano-SIM cards, and the standard flow is to activate the SIM through the au official app, the My au app, or by dialling the au customer centre from another phone with the SIM inserted.
Insert the nano-SIM tray into the handset with the power off, then power it on and wait for the phone to detect the card. If the device prompts for a PIN, the default codes are 1234 for the older au cards and 0000 for newer au nano-SIM cards. From there, open the au app on the phone and sign in or create an au ID account, then follow the SIM activation prompts on screen.
You will need your au account number, your date of birth and a mobile number for contact. If you are transferring an existing au number, the process moves from the old handset: insert the new SIM into the new phone and wait for the phone number to move across, which typically happens within a few hours of the device registering. Keeping the old SIM in the old phone powered on until the transfer completes speeds this up considerably.
If you are bringing a number from another Australian carrier, this is called a number transfer or porting. You need your new SIM to register on the network first, which usually takes a short period of waiting, then you submit the porting request through My au or the app, providing the losing carrier's account details, the number being moved and your new au billing information.
Common Problems and How to Resolve Them
A phone that shows "No SIM card" when the SIM is seated often points to a faulty tray rather than a registration failure. Check that the nano-SIM is not inserted upside down, that the tray's contacts are clean, and that you are using the tray designed for that specific model. Bent tray pins are a frequent culprit in second-hand devices.
If the phone displays "SIM not supported" or "invalid SIM," the card itself may be inactive, damaged or not yet activated. au SIMs that have sat unused for an extended period can be deactivated, and a SIM that has been reissued to another customer may simply be dead. au can reissue a fresh card at a store, though this means recreating any eSIM or au ID-linked settings on the new card.
Battery drain is another issue buyers often discover alongside the SIM switch, particularly with older handsets that have been through several owners. If the phone barely holds a charge while sitting idle on your new au connection, the cause is usually battery wear rather than the network configuration. This guide on fixing a phone battery that drains fast is worth reading before you decide whether the device is worth the trouble.
Registration loops, where the phone searches for the network endlessly, usually indicate weak signal or a band mismatch. au's coverage in regional Australia and outer suburban areas differs from the metropolitan footprint, and some international variants of a phone will only connect to a limited set of bands, meaning you get service in the CBD and nothing in the outer suburbs or beyond.
Where to Get Help and What It Should Cost
If you are near a NSC-operated mobile shop, the staff handle au SIM registration, plan changes and handset setup as part of their standard service. Because NSC runs authorised docomo, au, SoftBank and Y!mobile counters across Fukushima and Miyagi in Japan, they are also experienced with devices imported from the Japanese market and the carrier-lock problems that come with them. For readers in Australia who need an au-branded device that has never been locked or tied to a Japanese account, buying through an Australian au-authorised channel removes most of the risk described here.
Prices for the work are modest. A single au SIM typically costs a few tens of dollars, plans are billed monthly and are easy to change once the number is active, and the labour involved in setting up a new SIM, transferring data and moving your number is normally a short job rather than a technical project.
Should you want a device that skips all of this, the simplest route is to buy an unlocked handset from an Australian retailer and pair it with a prepaid au SIM. Prepaid plans require no credit check, no contract and no long-term commitment, which suits travellers, students and anyone who wants flexibility while working out their needs.
Comparing Your Options at a Glance
The main routes through the used phone and au SIM process differ in cost, speed and how much work you need to do yourself. The following comparison sets out the realistic differences between the common approaches an Australian buyer is likely to consider.
| Approach | Typical Cost | Time to Working Phone | Main Risk |
|---|---|---|---|
| Unlocked Australian handset plus prepaid au SIM | Handset price plus a small SIM fee | Same day | None significant; simplest option |
| Second-hand phone from a local reseller with unlock included | Device price plus activation fee | Hours to one day | Verify unlock status in writing |
| Japanese-market imported device with au SIM | Device plus SIM and possible unlocking service | Several days to two weeks | Carrier lock, missing LTE bands |
| Phone with an existing au number ported across | Device plus SIM, porting free with most plans | One to three days | Porting request needs accurate old carrier details |
| eSIM au plan on an eSIM-capable handset | Plan fee only | Minutes | Device must support eSIM and be unlocked |
Buying through an authorised channel, or having an importer handle the unlock before shipping, eliminates the two risks that most often derail an otherwise inexpensive second-hand purchase.
Get Your au SIM Working Today
Whether you are bringing a handset back from Japan, picking up a bargain at a local market in Melbourne, or simply replacing a SIM in a phone that has served you well for years, the path to a working au connection is clear. Check the bands and the lock status first, activate the SIM through the au app or customer centre, and allow your number transfer to complete before cancelling anything on the old account. Book an au-authorised counter for help if any step refuses to cooperate, and your used phone will be carrying an au SIM and a fully registered line the same day.