"I was running with the second-placed bloke and we both came into the Buried Village aid station [at 35km].
"I kind of blew through and didn't stop, and thought with 15km to go it is either now or never. I tried to empty the well as they say."
He said from there he just pushed hard and held on to win.
"That's just what happens in an ultra, you have to choose your timing and when to go, and do it right," he said.
Backhausen also won the 100km race at the Great Ocean Walk 100 in Australia last month.
Well-known Bay of Plenty runner Morrissey finished about nine minutes behind the Australian to claim silver.
Morrissey won the 42km distance at the inaugural Tarawera Trail Marathon and 50km event last year and stepped up to the 50km event this year.
Auckland's Sam Corbett rounded out the podium a further two minutes back.
In the marathon distance, Raglan's Tom Wellby won the men's race while Christchurch's Andrea Davies took home the women's title.
Results:
50km: Men: 1 Majell Backhausen (Aus) 4h 12m 21s, 2 Chris Morrissey (Tauranga) 4:22:56, 3 Sam Corbett (Auckland) 4:24:12. Women: Fiona Hayvice (Wellington) 4:46:03, Jo Johansen (Wellington) 4:47:07, Lucy Bartholomew (Aus) 5:05:09.
42km: Men: 1 Tom Wellby (Raglan) 4h 0m 35s, 2 Thompson Tapsell (Aus) 4:14:56, 3 Steve Alden (UK) 04:25:48. Women: 1 Andrea Davies (Christchurch) 4:35:21, 2 Nikola Fisken (Rotorua) 4:46:15, 3 Nicola Schnauer (Auckland) 4:49:11.