I live in Sydney now. Have done for 5 years with young family. Purchased a house in 2012 so I can live the landlord-free lifestyle and despite all the extra work involved in maintaining it, I do like owning over renting. However having a family plays a big part in that desire.
I like living here. There are good opportunities for IT workers although pay would be better in London. I like the climate and the people we have met are friendly. Seems a bit more laid back here.
I don't like the driving attitude, it is almost impossible to be let into another lane without forcing your way in. At least when I left the UK driving seemed more polite. The trains are slow but probably as a result much more reliable. The other day a train was 10 minutes late and I was shocked. It rarely happens here. I am talking about local trains, have no idea about the inter-city ones.
Less of a drinking culture. I.e. if you say no to having an alcoholic drink it wont bet met with 'whats wrong with you' or 'come aaaaan', which I think is a positive. It is silly the drinking culture in the UK and especially the pressure to get drunk.
Would recommend Sydney for raising a family. Especially at the younger age with the good weather and the outdoor things to do. At university age I think the choices are better in the UK though but there are some decent technology universities in Sydney.
It feels more multicultural here than in the UK although maybe comparable to London. The places I have worked have had people from all over the world and it seems every ethnicity has a presence here so I really like that now that I am used to it. However perhaps that is more of a city vs. country town thing than a UK/Australia thing.