Rents are very pricey, and if you live far out enough to have lower rents, the commute is much longer. We settled for high rent and low commute so dh could spend more time with us, actually enjoying being here iyswim. But he earns way more than the average wage. We claim back a lot of the rent on the 'Living Away From Home Allowance' because we are on 457 visas rather than having permanent residency. And even so, I find I'm having to watch our expenditure.
It's also worth thinking about medical costs, especially if you are planning to have the baby over here. If you're on a 457 visa (temporary residency, sponsored by employer) the UK and Australia have a 'reciprocal agreement' which means you are eligible for a temporary Medicare card. This covers certain essential medical costs but not things like ambulance cover, dentistry, or an independent midwife, or birth in a private hospital (if you were thinking of either of those last two). If you take out travel insurance, that will end when you arrive in the country.
You can't take out hospital cover if on a 457, except something called 'overseas visitors cover' or something like that which basically covers the same things as Medicare (designed for people from countries without the 'reciprocal agreement') but costs about $5k for a family! I've only recently found out about this aspect of it - I was looking into it because since dh earns above a certain salary he is actually taxed for not having health insurance, even though we aren't eligible to get it! Some companies pay for health insurance for their employees, others require you to cover it, so it's worth finding out what your dh's company is offering.
Also check the company is prepared to pay a fair bit towards relocation costs. The amount dh's company offered at first probably wasn't even enough to cover our flights, let alone anything else. In the end they paid enough to cover flights, 'removalists' (moving all our possessions from our 4 bed house) and some of our accommodation in Sydney when we first arrived. I think we still ended up out of pocket as we spent 4 nights in Hong Kong to break the journey and none of that was covered, but that was our choice. It's a permanent job, even if he's only on a temporary visa at the moment, so if we do end up moving back we'll have to cover the costs of returning to the UK ourselves.