Yes, and the reasons are outlined by the Australian Electoral Commission, the independent body that runs Australian elections (see the first FAQ)[0].
There are scrutineers that watch counting happen at the booth once polls close, and who also see and hear the numbers get phoned into HQ. HQ has more scrutineers from all parties checking both postal votes and recounts.
If anything doesn't match up it gets flagged. I think that the ability of every party to watch votes themselves means that trust is increased, and they have skin in the game (if they didn't object at the booth why not!?).
Pen markings are perfectly valid however, so you can bring a pen to the booth to vote with if you'd like to do so.
It's also true of course that erasers don't quite erase pencil. It would be fairly obvious that the paper was tampered with.