With regards to medicare on a 176, you don't need a job (if you are on the 176 - this isn't for temp. visas I don't know about those....), you just need the 176 to be eligible - when you land you go along to the medicare offices and get yourself registered and they give you a card (handy for ID points purposes).
You have a year to take out health insurance before the age premium thing kicks in =- basically once you hit 30 you start getting old age weighting on your health insurance premiums unless you took out your policy before the age of 30, ie it gets more expensive the older you get. To help out immigrants, you get that under 30 price for up to 12 months after you arrive, so your premiums are as if you took them out before you were 30 (obviously if you aren't 30 then that won't be an issue anyway). So you have a bit of time to work out whether you are going to take out health insurance or not. If you earn over a certain amount you have to pay a bit more tax towards Medicare if you don't have private healthcare.
There's some information here about medicare, reductions if you are on a low income on the tax (called the levy) and the surplus levy if you earn over a certain amount.
Ambulances aren't covered by Medicare, you WILL NEED ambulance cover (or get a massive bill if you ever need/use one, or your family does, for eg kids getting in an accident at school) - you can get this as part of private health insurance, or just on its own, or as part of extras private health insurance (basically, you use medicare for normal healthcare and have insurance for dental, ambulance and opticians for eg - there seem to be different plans....) have a look on a couple of health insurance websites for details of this basically - http://www.iselect.com.au/, http://www.medibank.com.au/ - there seem to be loads of them....google is your friend lol
Hope that all makes sense and helps a bit :)