A 2005 paper from Policy Exchange explains this dynamic in greater detail:
In the words of Kieron Barnes, senior planning officer at Adelaide Hills council, “The South Australia Labor government created an urban growth boundary around Adelaide three years ago  with the intention to stop the sprawl and to consolidate the city. But you could have guessed what happened then: People decided to move behind the growth boundary to places like Mt Barker from which they then commute to work in Adelaide. I was actually lucky to have bought my house there just before the growth boundary was put in place because after it was introduced land prices in Mt Barker soared.”
How did the state planners respond? “Well, now they have created more growth boundaries around the smaller cities as well to stop this kind of leapfrogging.”
Talking about his own personal house preference, he admits that he likes having a large house and does not mind commuting to work by car. Asked whether that was not actually contradicting planners’ beliefs in consolidation and promoting public transport, he smiles: “It’s difficult for planners not to behave hypocritically when it comes to personal choices. Many I know live in big houses on large parcels of land with two cars that are not necessarily environmentally or economically efficient.”
Of course, we should not be surprised by this outcome: similar phenomenon have occurred in other Australian cities.
The higher land prices arising from the imposition of an UGB tends to force many lower income households to ‘leapfrog’ the boundary and settle in far flung locations where housing is less unaffordable. In the process, UGBs can actually exacerbate urban sprawl and increase car reliance and energy usage, with particularly detrimental distributional impacts on lower socio-economic groups.
It’s yet another example of the perverse outcomes from urban growth constraints, which often have the opposite effect, thus eliminating many of their purported benefits (in addition to worsening housing affordability).