Hello, I am coming back to Thailand in November and want to stay for 6 months. I am visiting my father who lives there.

I what visa should I Get, I am from Australia.

Thank you.

What is your fathers status here?

For a 6 month stay a 2 entry tourist visa would give you about 6 months without a need for a new visa by extending each 60 day entry the visa allows by 30 days days at immigration,

You shouldn't have too much difficulty getting a double entry tourist visa if you get it in your home country. That will give you two 60 day entries. You can extend the first one for 30 days at immigration wherever in the country you are, then after 90 need to go to a border and pop out and back in to activate the second entry, after a further 60 days another 30 day extension at immigration.

The other option is a non-immigrant "O" visa which gives you 3 months and is then extendable without leaving the country and theoretically covers "visiting family" but this might be more difficult/complicated to obtain. No harm to ask the consulate about it, they are generally reasonably helpful.

my dad has a buisness in thailand, and has a thai wife.

Thanks for your help.

You could get a single entry non-o visa to visit family. It is even possible to to get a multiple entry non-o on the same basis plus the added fact the you are the step child of of a Thai.