Put your idea down on paper, written in the clearest most un-misinterpretable way, so that there can be no confusion.
Make 5 copies of said document.
Place each copy in a sealed envelope, criss-crossed with wax crayon to prove any tampering has not occurred, and on 5 consecutive days post each envelope to yourself by Registered Delivery (or whatever Pony Express security service you Aussie folk use these days).
The reasoning for the 5 copies is simple: it prevents anyone claiming to have thought about it 'at the same time', it precludes the remotest possibility that a single envelope might get lost, and it proves beyond any doubt that you were of cogent mind when you sent them.
When you receive each envelope, duly franked via the postal service (or trodden on by the Pony) you place all 5 envelopes in a secure place UNOPENED. A safe-deposit box in a Bank is by far the best option.
NOW you follow the PATENT route, which might prove fairly arduous, not to mention time-consuming and expensive.
You wouldn't be the first person to register for a patent just to find some snake-oil-merchant stole your idea from the Patent Office. The pre-dated envelopes in the Bank vault prevent this from happening. :)
