yes, there aren't any ATM's before immigration, that's why I said it would solve the issue of not having 20,000 baht, but that's not the real reason they want to deny entry .
Yes, about AUD 1,000. Not a great amount for a holiday. I know some don't like to carry a lot of cash on them, so maybe access to an ATM would work, but as you said, it's usually just a simple excuse to deny entry.