You can call industrial relations in your state- it's illegal.

But as you mentioned- what leaves you better off financially? Honestly if it was me, id just not turn up and cop a written warning.

When they give it to you, make sure it's in writing. You have the right to keep a record of conversation as well during the discussion, and have the other party (your manager) sign off on it as being an accurate representation of the discussion.

They're also required to give you 24 hours notice before having a performance discussion and you have the right to bring along any support person you like (from in or out of work).
If they breach any of that, just keep it documented and pull them up on it. They stop trying it on pretty fast if you sound like you know the HR side of their attempted 'performance management.'