Who controls the Prime Minister and his cabinet?

Well technically the queen / governor general, but the power they hold over our government is really long gone. So in reality, no one controls them.

Additional.

But if they perform badly they are likely to be chucked out at the next election. Individuals (Prime Ministers) can be removed by their own party, e.g. Margaret Thatcher, or the PM will remove a Cabinet Minister if he's not doing a good job. But they are not removed as Members of Parliament, only as functionaries within the Government.