How much passport validity do you need?

Not everywhere. It is the most common rule but it is not universal. Japan, the United Kingdom, Canada, Ireland, New Zealand and the United States all admit Australians on a passport that is simply valid for the stay. The Schengen countries want three months beyond the day you leave the area, not six from arrival. Check the country you are actually flying to.

It depends on the country, and the difference can be weeks. Most of Asia counts six months from the day you land. The Schengen countries count three months from the day you leave. South Africa counts 30 days from departure. A passport that clears one rule can fail another for the same trip.

The airline refuses to board you in Australia. Carriers check passport validity at check-in because they carry the cost of flying anyone home who is refused entry, so it is caught before you fly rather than at the border. Travel insurance does not generally cover a trip you could not start for this reason.

The Australian Passport Office quotes about six weeks for a standard adult renewal, and a priority service for an extra fee. Renew before you book anything non-refundable if you are close to a limit.

Rarely. The Schengen rule is that the passport was issued within the last ten years as well as being valid three months beyond departure. Australian adult passports are issued for ten years and children for five, so an unexpired Australian passport satisfies it automatically. It catches travellers on passports from countries that used to issue longer ones or add extension pages.