I'm not an expert: When I checked a few years ago , CGT liability in the UK also depends how long you live overseas. I think after 7 years (living outside the UK) , there is no CGT to pay in the UK if you sell a UK property. The Australian tax man is only interested in the UK capital gain FROM the time you moved to Australia till the time you sell the UK property.
e.g if you rented out your UK property from 2012 onwards, and still lived in Australia 7 yrs later in 2019 before selling the UK property, then Uk CGT = zero , and Australian CGT = UK house selling price in 2019 minus the UK house value in 2012 ( not at the time you originally purchased it ).
Note there was some vague small print in the UK CGT notes about there is a chance you may have to pay the UK CGT if you ever returned to the UK full time, ie its not zero its just differed ( but as someone said you avoid the UK CGT on a home for the first few years anyway so it shouldn't be too high)
I'd ask an expert or the Tax office to confirm the above as CGT laws seem to change especially in the UK . I've no idea how it works in reverse should you sell an Australian property but as you will be living in it there should be no Australian CGT.
One other observation. The current exchange rate to A$ is terrible for Brits at the moment so if you do sell your house and bring your pounds over here you won't get much for your money. The A$ will only weaken once Europe and USA economies pick up and/or China gets a cold. This might be years away or round the corner....