You don't need a visa for Hong Kong if you hold a UK or EU country passport.
I would book the flight first. If you are just going to Australia for a holiday you can apply for the visa on the internet, it doesn't take long and is free.You can stay up to 3months on each visit and the visa is good for 1 year after the date it is granted.
There are a lot of agencies on the net who will charge for obtaining a visa so make sure you use the correct Australian government site where the visa is free http://www.immi.gov.au/ click on the box in the upper right hand corner that say eVisitor and you will be able to fill in the form on line.

Thanks Judith, that is a brilliant help!

Good advice from Judith.

I have recently returned from holiday having done both Australia and Hong Kong as well. We flew through to SYD first but stopped in HKG for 4 days on the return.

Our Australia e-visitor visa's were very simple to apply for and were granted via email less than 24 hours later.

I want to go back !

Traveller wrote:

Good advice from Judith.

I have recently returned from holiday having done both Australia and Hong Kong as well. We flew through to SYD first but stopped in HKG for 4 days on the return.

Our Australia e-visitor visa's were very simple to apply for and were granted via email less than 24 hours later.

I want to go back ! :sun2

I think lots of people go to Australia regarding it as 'the trip of a lifetime' but it's just the start. I've been twice and can't wait to return. There is so much to see and do.