What unions? Union membership in Australia has been falling since at least the '70s (source). The press is almost always anti union resulting in some of them being legally dismantled when they ask for too many rights. Such as the BLF. The unions that remain today only ever go on strike as a last resort, when conditions are dire.

Regardless unions are able to go on strike because they take away something bosses need: workers. When workers go on strike profits for bosses fall immediately and bosses are forced to do what they need to get workers working again.

Politicians don't have this leverage. Political parties must do the bidding of capitalists in order to maintain donations to fund their advertising campaign. The capitalist class owns virtually all media and have almost complete control in terms of shaping peoples opinion on the current government. Politicians can't implement policies that benefits workers at the expense of the rich.

Not to mention authoritarian countries stand up to corporations they don’t support.

I'm going to assume by "authoritarian" you mean socialist countries such as Vietnam, China, Cuba etc. These countries have the ability to stand up to corporations because they are truly democratic. The press of our capitalist country (I'm Australian too) has told us these country's are authoritarian because they pose a social order that competes with capitalism.

The reality is that these countries are able to stand up to corporations because their government is ruled by the working class, not by the capitalist class. This is also why these countries consistently make decisions that benefit the working class at the expense of the capitalist class. Decisions such as cheap or free healthcare, immense poverty reduction schemes, increasing wages etc.