(If you just want to keep your Wii system normal)
Nintendo games are put onto DVD-ROM discs, Australia's and Thailand's DVD region codes are different, so it would not be compatible. Austrailia is in Region 4, Thailand is in region 3.
(If you want to Mod your Wii)
However, you could Mod your Wii, most mod chips are able to circumvent region coding and copy protection, which will let you play games created in different regions.
Here are some Pros and Cons of modding your system with a mod chip.
You can play back up copy's of your games.
You can play games from different regions (Such as you want to)
You can play homebrew games. (User created Wii games)
1.This WILL void your warranty
2.Some updates may try and Brick your Wii (Make system inoperable, you will have to buy a new one.), always read what people say about the new update before you update if you have a modded system.
3.Keeping your Wii up to date is very important especially when you are playing out of Region disc games, you could brick your system if you don't be careful.
To learn more about modding your Wii, go to this website.