If you are no longer resident or ordinarily resident in the UK you are not subject to capital gains tax on the disposal of an asset, so long as the disposal occurs in a UK tax year in which you are NEVER resident or o/r in the UK, and so long as you do not return to the UK within 5 complete tax years of your departure.
HM Revenue booklet IR20 confirms this (search at
http://www.hmrc.gov.uk).
If you are tax resident in Australia at the time of the disposal you then need to consider your liability to CGT in Australia ...
If you are interested there are also some free tax factsheets here:
http://www.collettandco.com/factsheet.cfm
Best regards.
__________________
Managing Director, Go Matilda,
http://www.gomatilda.com
Registered Migration Agent Number 0102534 and Chartered Accountant (England & Wales, and Australia)
Offices in the UK and in Australia