You have to turn up, get your name marked off the roll, walk over to the booth, submit the ballot paper into the box.
If you don't walk over to the booth you will be asked to do so. Whether you end up writing anything on the ballot paper is up to you.
You can pre-poll. You can use a postal vote. If you don't vote you will be asked if you have a valid excuse for not voting. If you don't have a valid excuse you will be fined. If you are marked off multiple times you will also be asked to explain and can be fined if it is determined that you voted multiple times.
If there are enough irregularities detected that the result could have changed count will be declared invalid and a new election held.
Recounts are automatic if the margin is below a threshold and can also be called for.