I've lived 20yrs Sydney, 18yrs London, 2 yrs living 2hrs outside London and another 3yrs elsewhere.
Sydney, of all Australian cities is expensive housing wise. More expensive than parts of London! One thing to look into is school fees. From watching that show 'Wanted down under' it seems that some people need to pay fees for schooling that locals don't. I would hope such fees are covered by DH's job, but worth looking into. It would be worth watching some of the wanted down under shows. I find that very few actually show moving to Sydney though- I think due to the increased costs to live there.
Also check what healthcare is provided by DH job- if any. Although not as horrendous as USA healthcare in terms of NEEDING to be insured, when I lived in Aus- it was pushing towards more private insurance. Things were covered on medicare (similar to NHS) but you got a tax exception for having private insurance. Ambulances weren't covered unless you had insurance and waits were longer if you went down the public route. Waits were no where near as long as the UK though- even for public healthcare.
Does the job come with any housing? There is a vast difference between the temperatures of living near the coast in Sydney, as opposed to inland near the base of the hills- despite this being less than a 1hr drive! Inland is FAR hotter and colder than the coast and has far more extremes. The house prices generally reflect this though. Inland tend to have more new build areas area- but there is new and old all over.
If you have an area in mind to live- how far is the job for DH? Although Sydney does have public transport, its nothing like central London. Majority of people have a car to commute, unless you are extremely central or near an easy train/bus route.
The al fresco lifestyle is great. Having friends over for a BBQ in the garden, eating out, swimming in the sea and work/life tends to be more relaxed. Internet shopping isn't as diverse or vast as the UK, but its improving.
I agree with renting out your Uk house. I recently looked into taking my dog to Sydney. I was advised that the whole process could take 8mths to organise due to rabies vaccination, testing, further papers and vet checks. I can't recall exactly, but was quoted several thousand £'s for a 6kg dog! That was one way. They said I'd need to find an Australia transfer company for the return journey. IF you are considering taking pets- look into that NOW!
Best of luck with whatever you choose to do. Its a fantastic opportunity, and if you can rent the house it- you have somewhere to return if needed.