I am a UK citizen - both entries of my double entry visa are used but I want to go out & back again

I am staying with family who are working here and helping with child care ... I've had a 6 month double entry but have used both entries. The family are going on holiday in october to Australia & I want to go with them. What is my best option to re-enter? Do I need a new one and will I have to go to Hong Kong for it before going on the holiday or should I just do it while there in Sydney?