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I think Cambodia first is the better option, then Vietnam before Aus. I guess the only thing I really need to work out is how long for each leg. I'm assuming a week and a half in Cambodia, then the same again in Vietnam should be enough??
Cheers again for this, you're making my life much easier 
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Hi,
A week and a half = around 10 days for each country, that sounds nice. Personally, i will divide time as below:
In Cambodia:
3 - 4 days for Sieam Riep: Angkor Wat, Angkor Thom and other temples
2 days enjoying beaches and seafood in Sihanoukville
2 days for Phnom Penh: Royal Palace, National Museum, Tuol Sleng Museum, Russian Market, Wat Phnom, Killing field
Total: 7 - 8 days
In Vietnam:
2 days for Ho Chi Minh city: museums & Cu Chi tunnel
4 days in the Centre
Da Nang: nothing much to see, except Ba Na mountain & Five Marble mountain (on the way from Da Nang airport to Hoi An), just half an hour you can explore this mountain, only take 2 days for Five Marble mountain & Hoi An ancient town (Hoi An, better to come on full moon day, you have chance to join many interesting activities for tourists)
Hue - 2 days: Ancient Citadel, Royal Tombs, Thien Mu Pagoda, Lang Co Beach, Perfume River.
6 days in the North: Ha Noi - 1, Ha Long bay - 2, Sapa - 3 (8-hour train from Ha Noi to Lao Cai and vice versa)
Total: 12 days
3 weeks = 7 x 3 = 21 days, spend 20 days and one day left for resting before flying to Australia.
I skip some places in Vietnam, just show you most-visited places. With this route, you may travel lots, but hope you enjoy your trip at the most.
Cheers,