We arrived in September and did manage to secure a 12 month rental before we left the UK. It was a bit of a gamble, but we checked out the area using Google Maps and just hoped the house resembled the agents pictures, which it did thankfully. We did offer to pay 3 months rent up front, but I also had a job lined up here which I think helped as I know they telephoned them to check my references were okay. Our rental is through Toop and Toop, so it might be worth giving them a try if you are still keen to get something sorted before you arrive. We are renting in Grange (a nice beach side suburb) but have now bought a house so will be breaking our lease in March which looks like it will be too early for you, but if anyone else is interested then send me a pm.
Best of luck with your plans.