Before 25 February there is no verification of ETAs by the UK government, only manually by airline staff.
Some people born in Aus to UK-born parents are automatically British citizens, while others aren't, depending on their precise circumstances.
As an example, if you're born before 1 July 2006 to a UK-born father who wasn't married to your non-British mother, you are not an automatic citizen. While if born after 1 Jan 1983 to a UK-born mother and a non-British father, you are an automatic citizen whether your parents were married or not. For some people, determining whether they are citizens can get incredibly complex.
The UK won't know that someone is an automatic citizen until they attempt to apply for a British passport for the first time, unless the ETA application is rewritten to ask the same questions as a first passport application.
If an automatic citizen has never tried to get a British passport, and they apply for an ETA saying they are not a British citizen (which could be an honest but wrong belief, or a deliberate lie), it's highly unlikely those ETAs will get cancelled. IMO, there won't be any problems using that ETA even after 25 February.
If you are not an automatic citizen you may have a shorter route to obtaining British citizenship, but there is no obligation to do this and if you don't do it, you will be treated the same as any other Aus citizen when getting an ETA.
If you've previously had a British passport, it seems that the Home Office has no records of passports issued prior to around 1995, so relies on physical evidence submitted by applicants when they try to reapply for British passports. It's unknown whether the ETA verification system will be linked to data of recently expired passports.
If you are an Irish citizen, you're meant to use an Irish passport and not get an ETA to travel to the UK, but the UK would generally not be able to know whether someone is an Irish citizen. Technically, everyone born in Northern Ireland and the Republic of Ireland before 2005 is entitled to be an Irish citizen, but they are not Irish citizens until the first time they make an application for an Irish passport (or, apparently, enrol to vote in an Irish election for the first time).