由香港去澳洲要幾小時和幾錢?

That depence on which part of Australia you are planning to fly to:
Hong Kong-Adelaide around 8hours 30mins
Hong Kong-Brisbane around 8hours
Hong Kong-Cairns around 8hours
Hong Kong-Melbourne around 9hours
Hong Kong-Perth around 8hours
Hong Kong-Sydney around 9hours 30mins

For non-stop flights, you can choose Cathay Pacific to fly to all destinations listed above, QANTAS to Melbourne/Perth/Sydney but can transit to other destinations across Australia, Virgin Atlantic to Sydney but can transit to other destinations across Australia in Virgin Blue network...
And for price, it vary between low season and peak season... The lowest Virgin do sometimes is around HKD4000 during promotion... But during peak season, the price could be more than doubled... Both excluding tax...

TO:Adelaide around 8hours 30mins
TO:Brisbane around 8hours
TO:Cairns around 8hours
TO:Melbourne around 9hours
TO:Perth around 8hours
TO:Sydney around 9hours 30mins

when, and where?

香港乘飛機去澳洲約八個半小時