The further out you get the more land there is and the cheaper it is. However lots of people want to live near the city but there all the land has already been built on so there isn't much spare land. You then have to resort to building on brownfield sites such as Cheltenham racecourse, Underdale (which was a tertiary college), or subdividing existing properties, which seems to have got out of control particularly in the central beach suburbs. Whole streets are barely recognisable from what they were like 15-20 years ago.
So property in some areas is much cheaper than the UK (the median house price in Elizabeth is only $190,000) but in some areas it isn't cheap at all (Burnside $655,000, Rose Park $1,040,000, St Peters $860,000).