Are you sure you're not going over on a 457 visa? That's the normal temp visa, I'm not quite sure what the 475 is all about.....
Assuming you're talking about a normal 457.
1. If you're on that sort of temporary visa you will need private
health insurance because you will not be covered at all in Australia. The employer may pay for the costs of private health for you, but more often than not they will build it into the contract so that you are the one responsible for the costs. Either way, you will need private health care which is pricey.
As far as other sorts of insurance they might be provided along with your superannuatinon (or super funds for short). This is like your RRSP's in Canada only here by law the employer must pay 9% of your salary into your super fund. If you're coming over on a temporary visa then you should be able to get it back when you leave Australia. We are permanent residents so we can't do that, if we leave it stays here and we can get to it once we reach retirement age.
Now, for example, the company my husband works has some additional insurance provided along with the super. These benefits are things like life insurance, disability insurance, long term illness, etc. So for us we get it through his company and the people they use to do their super funds. I should mention that I think we pay a small monthly fee for it. So either way it's probably up to you, although it's general best (cheaper, more benefits) if it can be done through work. If you don't have that option then it's your decision if you want to take out additional insurance.
While you are over on your temp visa it is possible to change jobs and get a new 457. For example, if you were fired or something you'd have 28 days to find a new employer to sponsor you or you'd have to leave the country (or so they say). However, if you just don't like you job and want a new one there is nothing stopping you from job hunting if you want. Once you've found a new employer to sponsor you then you'd need a new 457 visa.
It's easier said than done though for some people. I've heard of whole families coming over and then the sponsored person loses their job and can't find another one. So the whole family has to pack up and leave. So, in short, nothing is guaranteed on a temp visa and there will always be a certain element of risk.
I should also mention that if you do come over on a 457 and intend to stay for good then you should apply for a permanent visa (175) if eligible. If you are already on a 457 then you would stay on that visa until your permanent visa is granted. If you were to do that you'd have the benefit of being eligible for Medicare (the "free" public system) as soon as you apply for a permanent visa. This of course only works if you are already in Oz on a temp one.