As a professional groomer I have to agree with Duncan, and not recommend that an Aussie be shaved. When you shave down an Aussie the top coat or guard furs will become compromised and the regrowth will become softer and look more like puppy fluff over time, lighten up in color, and often loose it's shine. The reason being that the guard hairs are semi hollow and once you break the shaft the fur will loose its harsh protective texture. It may not happen the first time that you shave your Aussie, but over time the damage will become more and more apparent and it is almost always irreversible.
If you are worried about your Aussie becoming too warm you can always bring the dog to a groomer and have them clean out the undercoat and do a little trim with shears on the longer fur to tidy any of the longer areas up. Once the undercoat is removed the guard coat will actually help keep the dog cooler because it will act as an insulator against the heat.
If you are still determinded to shave down an Aussie I agree with D'tails, Andis makes a great clipper. With such a thick coat anything less than a professional grade clipper might pull the fur and leave the fur looking very choppy.