Hi Stuart Chittenden,
A De Facto relationship in Australia has no bearing on our CGT rules, it just gives non married individuals living together certain rights under Australian legislation .
If the two individuals have beneficial ownership of an overseas property, they are UK resident and sell that property then they must report any gain arising on their share in the property and claim foreign tax relief if appropriate in the normal way.
Please see :
Report and pay your Capital Gains Tax
Thank you.