Brad Schimanski (21), of Queenstown, reached the Kawarau bridge, in Frankton, on Thursday, just before 3am.
He had battled "a world of pain", the fast-flowing Dart River, sharp bracken on Cecil Peak, the 1805m high Mt Dick, and night-time highway traffic to complete the circuit.
"About 13km out from Queenstown, my support crew turned back up to run with me," he told the Otago Daily Times.
"I was crawling at that point. Just having fresh legs beside me, I got this buzz and I broke out into a run.
"Somebody said if I kept the pace up I'd break 70 hours and I got a big burst of energy."
The Ultimate Hikes guide and Queenstown volunteer firefighter aimed to complete his sponsored circumnavigation of the third largest lake in New Zealand in 48 hours, to raise funds for families in need in the community.
Mr Schimanski began his journey, called "The Wakamishu", at the bridge on Monday, at 5.07am, with his core Queenstown supporters - brother Karl Schimanski, girlfriend Raenee Wilson and friend Travis Donoghue - plus well-wishers.
One pal ran with him to Queenstown and another mate to about 30km along Glenorchy Rd, then he pounded the tarmac solo to Glenorchy and the Dart River.
"I chucked some boardies on and started wading through the braids soon after noon, for half an hour. It was mostly up to my knees or waist. I got to the main section where you swim and it was flowing fast.
"It took about half an hour to warm up in the car with the heaters on."
He changed into his thermals and beanie, donned his daypack and headed from Kinloch to Greenstone Station with Ms Wilson.
Five supporters had sailed on TSS Earnslaw to cheer them by the time they reached Mt Nicholas Station, at about 8pm.
Walter Peak Station staff fed the couple and let them stay overnight, then they departed on Tuesday, at 4am.
They reached Table Bay, at the foot of Cecil Peak, at 5.30am, and it was there difficulties began.
"It took about 12 hours to go 10km. We were thrashing through matagouri, getting cut up and I was bleeding."
It was only when they escaped the scrub and looked behind them that they saw the 4WD track they could have taken, he said.
"We struggled around Cecil Peak Station and Halfway Bay Station, but it was getting dark. We jumped in the tent at the base of Mt Dick.
"Raenee was a trouper. If it hadn't been for Raenee I wouldn't have got through."
After a 7am start, on Wednesday, they trekked up the windswept ridgeline of Mt Dick.
Mr Schimanski's brother met them with much-needed food supplies and the trio reached the summit at 3.30pm.
They headed down the 4WD track to lake level, where the terrain allowed Mr Schimanski to move at speed again.
He changed his socks and shoes again in Kingston and departed at 9pm.
He and Ms Wilson power-walked along State Highway 6, wearing high-visibility gear, and made up huge ground, he said.
"It was incredibly painful. My knee had started to swell up, so I had that strapped up.
My ankle was twisting inwards and any jolts would send excruciating pain through my body."
Ms Wilson rode in the support car, as Mr Schimanski broke into a run for the final stretch with supporters.
Mr Schimanski said he did not finish the trek "in the planned time, but it's still a pretty big achievement to do it in under three days and I'm ecstatic."
Mr Schimanski said he would be collecting sponsorships and liaising with the Salvation Army to find out how much he had raised online.
"There's been talk along the way to make it an event with 10 teams running around the lake - maybe a relay," Mr Schimanski said. There were many possibilities, he said.
