What are the limits for spending days in Thailand on a visa-free entry for Australians?

There are no limits, how many time you wanna come to visit or travel to Thailand if you have visa.

If you have Australian passport you don't get visa on arrival you receive 30 days visa exempt stamp which is zero visa just a stamp in you can get one 30 day extension whilst in the Kingdom for 1900 baht or get 60 day tourist visa issued in Australia and than 30 day extension give you 90 days or border crossing by land get another 30 day exempt entry but you are only allowed 2 land entries per year but there is no set regulation or law for the number of times one can visit the kingdom by air but any entry to Thailand is at the decresion of the IO infront of you at time of entry whether it be you first time or your 20th time it is looked at as if you are a tourist or you are trying to live there without the correct visa get correct visa for your purpose and you will have minimal issues Chokh di

No limit at all. Maximum two border crossings per calendar year, no official limit on flights, but if you're doing back to back entries eventually you'll be interrogated and possibly denied entry. The pattern you describe is fine, you'll have no problems

There's actually no such thing as a 30-day Visa On Arrival. You're actually entering with no visa, just a free 30-day entry stamp. There's no official limit, the only instances where immigration will pay close attention is when people are doing back to back border bounces to remain in the country for long periods with no visa. Many FIFO workers use the free stamp with no problem, because they're in and out the whole time and are not spending long uninterrupted periods in Thailand. If you're coming three times a year, spread out through the year, you'll have no issues

No

There is no written law or regulation that states you can only spend a certain number of days in Thailand.

The final decision on your entry is at the discretion of the immigration officer. They will look at your history in Thailand and think if you are actually a tourist or not based on this history. If you are entering for 30 days, extending that for 30 days, leaving for 1 day, 1 week, and immediately returning for 30 days, extending for 30 days, and repeating this cycle, it looks like you're not a tourist and you are trying to live in Thailand.

If you fly to Thailand, spend a month, visit some other countries, go back to Australia for 2-3 months, then go back to Thailand for a month, that is not a pattern that is suspicious at all. That looks like a tourist who really likes visiting Thailand and you shouldn't encounter any issues.