Can Australians apply for a 90-day visa on arrival in Thailand, or do they need to acquire it beforehand?

no matter what visa you come in on the amount of time you're admitted starts from the date you arrive in thailand.

Where as the validity of a VISA (that you get from a thai consulate) starts counting down the second they stamp it into your passport..

But the 60day visa, as described by tod, can be extended in country for a further 30 days (1900b), giving you 90 days total.

The only visas that get you stamped in for 90 days are Non-Immigrant Type ones. You get them based on being married to a thai, being over 50, raising children here, volunteering, working, studying, etc.

Australians can't by ANY visas to thailand when they arrive here and there is no such thing as s standard 30 day tourist visa :/

When you came in last time you entered on what's called a 30 day visa exempt entry. <- it has that name because you didn't' BUY a visa from a thai consulate before you got here but were stamped in for 30 days without one.:)

You can get a single entry tourist visa from a thai consulate before you come here and that will get you stamped in for 60 days.