I did this last year and am planning on going back and doing a second year next summer with a mate. Id recommend it to anyone.
I found work easy enough to find if you know where to look. If your heading up the east coast, check out the Maroochydore YHA. Its a small(ish) beach town, really nice though with everything that you need there (cheap bars, shopping centre, cinema etc) and the hostel was the best I stayed in during my travels (and there were a few shocking ones). Met some amazing people there, who I still talk to and will no doubt keep in touch with. Feels more like one big family than a hostel when your all working and living together there. Honestly it was one of the best experiences ive had, to work your arse off during the day together, the camorarderie that you develop, to then get home, crack open a cold beer and sit in the hammock by the pool, then later on cook a big roast for about 20 of you. So chilled out it was unreal. Happy days.
Anyway, im rambling. Basically the hostel has links with one of the biggest strawberry farms in the area so your pretty much guarenteed work if your staying here (the farm takes in well over 100 pickers and packers). You might have to wait a week or two depending when you arrive and how many they are taking in a week, but with people leaving all the time and moving on, you will get work. The season there last year ran from something like February through to the end of August, with July and August being peak. I was earning $17.50 an hour there and in three months myself and my partner at the time saved over £5000 between us.
Strawberry picking is hard work (imagine touching your toes for 10 hours a day) so dont be expecting to be in a real life yoghurt commercial - like I did - but Id swim back there tomorrow if i could. Just an amazing experience and met so many amazing people. Because the farm was so big there were new people starting all the time as well so always interesting conversations to be had while you were there.
If you want it easier though, try and get on a ginger farm (they have links with a couple of these too and the seasons are different). Easy as anything. Imagine sitting on a bucket in the sun, just picking ginger out of a pile of soil whilst chatting away. No pressure. Easy life.
Along with farm work, the YHA also had a number of local businesses that called them if they needed casual workers, from turfing, working on local building sites just doing some labouring (or specialist work if your qualified), even green keeping on a golf course.
I obviously dont know if your going up that way, but like I said, id highly recommend that place.
To answer your question regarding backhand deals with farmers to sign you off, I know one or two tried that on the strawberry farm I mentioned and were given short shrift. On the other hand though, we stayed at Port Macquarie backpackers (in Port Macquarie, about 6 or 7 hours up the coast from Sydney) and the owner there offered us work in the hostel (assistant manager and cleaner) and he told us he could get us signed up by a local farm if we did 3 months for him. We didnt take him up, but it proves that it is possible. Id say be careful though, especially if your staying for the two years straight.
There was also a couple of Manc lads we stayed with who were there on second year visas that did one single day on a farm and just applied for the second year on the off chance and werent checked. Luck of the draw really.
Anyway, hope that helps in some way mate.