Backhausen entered the race after a few friends showed him a video of the scenic course.
"The course is amazing - it's absolutely stunning and I now hope to be back for the Tarawera Ultra in February 2016."
Hayvice said she was equally stoked with the women's win. She was hoping for a top three placing but didn't expect to come away with first place.
"It's a magic feeling," Hayvice said. "I've done the Tarawera Ultra three times and I wanted to try out this event with it's shorter distance and different trails.
"The atmosphere has been unreal and the start line at the geyser was incredible."
In the Marathon event, Tom Wellby from Raglan took the men's win in 04:00:35, while Andrea Davies from Christchurch won the women's race in 04:35:21.
Event organiser Paul Charteris said he was absolutely thrilled and he loved the finish line at Hot Water Beach, which was a sea of positivity and jubilation.
"I couldn't ask for a better day - though it has been hot and tough for the runners. So many people from Rotorua and beyond had no idea that Hot Water Beach even existed and they're loving it.
"For many this event is the biggest thing they've ever done."
The spectacular point-to-point geyser to volcano race started in front of the world-famous Pohutu Geyser in Te Puia's Whakarewarewa Valley.
A spine tingling haka bid the runners farewell and good luck as they set off to retrace one of New Zealand's most important historical routes - that of the Tuhourangi people after the Mt Tarawera eruption in 1886.
The course weaved its way alongside four lakes with stunning views, through an archaeological site at Rotorua's Buried Village, under a waterfall, through native and exotic forest.
The finish for competitors was on the shores of Lake Tarawera, where they dived into the lake water and rested aching muscles in Hot Water Beach waters with a cold beer in hand.
Boats and buses then ferried competitors and their supporters back through beautiful scenery to the start.
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 (marathon):
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.