OK, so if she goes to Australia on her own and applies for a 90 day single entry from there and then returns to Thailand this would be acceptable. Just confirming that I do not need to go to Australia as well?
Be aware that if you request a credit card from the same bank that you may have your THB800k in, they will, usually without your knowledge, freeze the same amount of your credit limit of your 800k. So when you go to extend your OA it will be denied as part of that 800k will be frozen by the bank!