I am flying from Australia to UK, changing planes in Ho Chi Minh City then Hanoi. Do I need a visa? Wasn't sure as the HCM to Hanoi is domestic.
As an Australian, you will need a visa in order to make this trip.
When you arrive in HCMC, you'll enter Vietnam in order to make the domestic flight up to Hanoi. In Hanoi, you'll then exit Vietnam in order to fly onwards to London.
If you're returning to Australia via Hanoi and HCMC, you'll need a multiple-entry visa - or a second single entry visa if you're not coming back for more than 3 months.
Vietnam Airlines also fly from HCMC to London, so if they offer it as an option you could look into changing your flight to London to depart from HCMC. This'll probably mean arriving on a different day.
Information on visas for Australians is available from the Vietnamese Embassy in Canberra.
An old Tripadvisor thread suggests that Vietnam Airlines might well be able to sort you out a visa for free if you contact them, although it may only apply to travel agent bookings - they don't seem to advertise this on their website, just on some travel industry material. Might still be worth contacting them though.
As I know, Vietnam transit visa is not required regardless of your nationality if transiting through Vietnam airports without crossing the immigration area. The transit visa is issued for tour escort accompanied by a tour guide. Source from http://www.vietnamvisa-easy.com/vietnam-visa-information/vietnam-visa-types.