How do you get a tourist visa extension in Australia?

If you want to stay in Australia longer than your original visa lets you, you will need to apply for a further visa of the same kind or another visa.

How much longer you can stay in Australia will depend on the visa that you have, or the visa that you apply for.

Visitor visa subclass 600 and the 8503 ‘no further stay’ condition

If you have a ‘No further stay’ condition on your visa, you cannot apply for a new visa while you are in Australia. You must leave Australia no later than the date on which your visa ends, except in extremely limited circumstances.