Hi Bluepope,
What you require is health insurance that meets condition 8501.
Basic holiday insurance doesn't meet this requirement and while I have heard one or two doing it in the past, its highly risky as it doesn't meet the DIBP requirements.
There are five main funds (such as HIF that BestyBlue discussed in this forum) that are compliance with the 457 visa and you should consider.
Two elements to consider:
1) There are waiting periods when it comes to health insurance in Australia. This means that if you for example take up extra's (eg Dental) which you are considering, then you will need to wait a period before you can see a dentist or get a dental procedure (major dental). This can range from 2 months to 12 months and is normally the same across the funds. If your stay is not planned to be long it may be worth not getting extra's cover. If on the other hand you plan to work/stay in Australia for a while, it may be worth getting started with these waiting periods so you can claim later for the family.
2) Health Insurance in Australia is highly regulated. This means that its easy to change from one health fund to another without penalties (or restarting waiting periods). This means its easy to switch from one policy (or fund to another).
I made a comparison site that has all the 457 visa health funds if you Google "457 visa compared" but otherwise you can just Google "457 health insurance" and see some of the main players.
Good luck!