If you are not a tax resident of Australia then you do not receive the tax free threshold and pay a minimum of 44 cents in the $ tax. Much more than in Thailand. And the tax authorities in Thailand do not know how much you are taxed in Australia, So I expect that Thailand will tax you at 15% or at 0.
First, it depends on your country of passport. If you paid tax on that pension in your home counrty and that country has a dual tax agreement with Thailand (like USA, Great Britain, Australia, New ZSealand and many other countries) then it will not be subject to Taxation by Thailand.
Before you can get a maariage visa (from a Thai Embassy/consulate) outside of Thailand or an extension of stay based on being married to a Thai (from an Immigration office inside of Thailand) you need to have your Thai marriage recognised by Thai authouities. Getting married in Thailand is a complicated process. The first step is to get a letter from your embassy stating that you are free to marry.
You must be legally married before applying for a marriage visa from outside of Thailand or for an extension of stay based on marriage from inside Thailand.