Basically, an ETA visa allows the foreign citizen to stay in Australia for a maximum 3 months at a time, although it is valid for 12 months starting the date of issue, or until your passport’s expiry date—whichever is sooner.
Having said this, the only thing you need to do is let your ETA expire and reapply for a new one in case your travel date does not correspond with the validity of your original ETA.
However, you should be reminded that you can apply for an ETA offshore, or when you’re outside Australia. That’s why the Australian government advises ETA holders to leave the country immediately before your passport and ETA expires to avoid problems at the airport.
Whilst in Australia, you can apply for a bridging visa