Where are you located in Australia? You generally want to plant garlic a good two months before your average killing frost date. Meaning the date when there is a 50% chance of a killing freeze.
They are absolutely useful. All you have to do is cut yourself once and you've learned never to do it again, learn to use the guard. Like Suzanne, I recommend the Benriner which is well made with a variety of blades though I don't...
Any mint can be invasive.
Sink a gallon pot in the ground with two inches of the pot above soil level. Fill with two to three inches of gravel, then potting soil (not top soil) and plant the mint. It will be much more easily contained.