Assuming you were/are domiciled in the UK, you are outside the scope of capital gains tax in the UK:
- once you have ceased to be tax resident and ordinarily resident in the UK, and
- if the disposal of a chargeable asset (such as real estate or shares in a business) takes place in a UK tax year in which you are NEVER resident and NEVER ordinarily resident
so long as you don't return to the UK within five complete tax years of your original departure. If you do return to the UK within that timeframe you could find that the capital gain becomes assessable in the tax year of your return.
You also have to consider capital gains tax in Australia, which is usually computed with reference to the value of your CGT assets on the date you become a tax resident of Australia.
Note that the existence of CGT tapering relief in the UK can remove fairly significant amounts of capital gains from the charge to tax, as can the Australian tax treatment of re-basing CGT assets to their value on the date you become a tax resident.
The free tax factsheets here might also help:
http://www.collettandco.com/factsheet.cfmHope that helps for starters.
Best regards.