Type of Visa required

We will be visiting Vietnam as tourists in March, 6th to 22nd of March and our last 5 days will be in Cambodia before returning to Hanoi for our return flight to Australia on the 26th March. We will only be transiting through the airport, getting off plane from Siem Reap and transferring to board our return flight to Australia and will not be exiting the airport. Do we need a 1 month single visit visa or a multi visit visa?
Steve