I'm from Australia Sydney too.
Photoperiod plants require a change in shorter daylight hours to begin to produce buds. So if you're growing outdoors you need somewhere between 16-18 hours of daylight for at least 1-2 months to let the plant go into its vegetative state, this is just where the plant produces leaves,stems & whatnot to further production of buds (known as the flowering stage). When the days get shorter the plant begins to produce bud colas and pack on weight before its harvested.
Autoflowering plants are a god-send. They do not require any changes in light to flower. Autoflowering plants have a timed-hormone mechanism - this just means that when the plant reaches a certain size (beyond our control, we can't tell how big it's going to get before it flowers) it will simply begin to produce buds that we can harvest. Autoflowers are a god-send to Australian growers because we have no frost here or snow which means we can do 4-6 grows a year easily. Some autoflowers can yield 200+ grams while others can only yield 20-50 grams. The issue with autoflowers is you will need to get yourself a Indica dominant strain to tolerate some of the heat we get during the summer, for winter grows you can do both Indica & Sativa dominant strains. I prefer Indica as it generally is easier to grow and yields more.
As for medium. I suggest you do not plant directly into the soil. Most/all of Australian soil is highly acidic and will kill marijuana. You should purchase some high quality soil, mixed with perlite and make sure the soil has no added fertilizers or time-release nutrients.
For nutrients i highly recommend Seasol Powerfeed (Can be purchased from your local Bunnings). You should start the plant on 1/8th of the recommend strength, then work your way up to 1/4th and 1/2 then 1/1 (all 1-2 weeks apart).
In-relation to your pot-size. 20 gallon is excellent. It will give your autoflowers (or photoperiod) plants enough space to make an excellent and strong root-system. If you're growing autoflowers in that much of a size, expect a 130-160 day grow. Bigger root system = longer for vegetative growth. But in the end, bigger root system = more buds = higher yield.