there is a similar thread on the forum if you do a quick search - almost exactly the same question in fact.
But to ease concerns - go out and ask how many employees would like to leave their job and do 'their own thing'. The response will be that many say they'd like to....and 2, 5, 10 years down the track they are still where they are.
There are many reason why and John S has listed a few - another is $.
The people who are likely to steal the idea are out there exploring every avenue anyway.
Discussing with an accountant for instance in my view is pretty safe.
But if you need to borrow and need venture capital or even a loan, well you are going to have to discuss it.
That said, it is worth spending a few hours reading the
www.ipaustralia.gov.au website. This will give you a heads up on what is protectable and what is not, and also how not to tread on toes for trademark infringement, inventions etc.