Sort of, remember that lots of people who arrive on 457 visas want to stay but cannot do so. If their employer refuses to sponsor, or they get refused on health grounds, or something.
You cannot 'migrate' to Australia on a 457. You are effectively little more than a visitor on a 457 visa. Unless and until, you are granted PR, Australia will expect you to go home whenever your employment ends.
457 visas are granted quickly, but so are employer sponsored PR visas.
Also - if you're going to Perth then visit options.
The exact visa requirements depend on the visa in question. Different visas have different rules. But in general, if your partner's German, he will need to do IELTS.
If radiographer is registrable, you'll need WA state registration before you are allowed to work.
Also bear in mind that although you will have work rights on a 457 visa, many employers prefer to hire PRs-citizens for long-term, career orientated jobs. In other words, many of the better jobs are closed to 457 holders.
You should check whether non-PRs-citizens are eligible for state registration as a radiographer in WA, if planning to work there.
If you had PR at the time the baby is born, it would be an Australian citizen automatically. No need for any visa.
If you don't have PR, it will automatically be granted the parent's visa.
Out of interest, how did you get your British citizenship? If you are British "by descent" your child will not automatically be British if born in Australia. You may be able to register it as British within a time limit, depending on circumstances.
-- This is not intended to be legal or professional advice in any jurisdiction
