Long time lurker, first time poster. We came out in 2008 and I've been trucking ever since.
Your car and motor bike licences will just be swapped over. Class I equivalent is called HC (heavy combination). Even though I had held my Class One since 1989 I couldn't just go for the HC licence. I had to pass a short theory test to obtain a provisional HR (heavy rigid) licence. I would strongly recommend an "unrestricted" licence, which means you have to know how to work an eighteen speed, twin splitter with double de-clutching. A lot of the big fleets are moving to AMT's, however for long haul and American based rigs the Road Ranger box still reigns supreme. Once you've passed your HR licence, you "officially" have to wait twelve months before you can step up to the HC or MC (road train) licences. However, since I'd had my C One since 1989 the licensing office waived this period. I have also heard of people being able to go straight for HC licences. I seems to depend on the discretion of the staff at the licensing office. In 2008 a one hour lesson on an HR truck was AU$40, HC AU$45, but had to be taken as two hour blocks. An MC assessment in 2012 cost me AU$ 900 for half a day, including theory. Dangerous Goods licence required to cart fuel takes two days, approx AU$400. For general freight a fork lift licence will be an advantage since there is a lot of self loading/un-loading, especially in more rural areas. You'll probably also need a fatigue management cert, the most basic one is free and can be found here: http://fatigue.safetyline.wa.gov.au/ Remember, ALL work related expenses are tax deductible.
Working hours can be a lot longer than in the UK. You can drive up to 17 hours per day! I've done a couple of 15 + hour days: ie six deliveries from Perth to Kalgoorlie over 1100 + kms, but paid nearly AU$900 for the day! However, working hours are supposed to average out at a max of 14 hrs per day.
To get your foot in the door I'd suggest TOLL through seek.com.au. They do most of the super market deliveries. I found it quite hard to even get an interview initially since a lot of employers ask for Perth Metro experience, but one will just have to persevere.
As for it being poorly paid: I work an approx 11 hour day carting bricks around Perth with only the occasional Saturday thrown in and have NO complaints.