Tauranga's Mahe Drysdale has held on and endured a nerve-wracking wait for the result of a photo-finish to win back-to-back Olympic gold medals in Rio de Janeiro.

Drysdale held off Croatian Damir Martin in a tense finish at Rodrigo de Freitas lagoon on Saturday (Sunday NZ Time), winning by half a bow ball as both racers were credited with the same time of 6 minutes 41.34 seconds, an Olympic-best.

Mahe Drysdale in action during is gold medal win in Rio. Image: YouTube.

Women's single sculler Emma Twigg and the women's and men's eights were unable to add to New Zealand's medal haul in their finals on the last day of rowing in Rio.

Twigg and the women's eight were unable to make a charge in the final 500m, both finishing fourth, while the men's eight made a promising start before fading to sixth.

Drysdale made his move in the third 500m, overtaking early pacesetter Martin as the pair left Ondrej Synek in their wake and battled for gold says Stuff.

The Kiwi, who won the same event in London four years ago, took an almost boat-length lead at one point before Martin pushed back and looked to have got over Drysdale in the final stretch. However, Drysdale put in a couple of big strokes to the line, winning by the narrowest of margins.

"That was absolutely amazing," Drysdale says. "It could have gone either way."

The race had been billed as a duel between the 37-year-old Kiwi and his friend and arch-rival, the Czech Synek.

But as it got underway, Martin was determined to seize the day, taking an early lead, chased by Drysdale and Synek.

The New Zealander moved ahead coming into the last section. He looked to be pulling away but Martin fought back and with a last lunge appeared to have edged it.

"With about 400 to go, I was feeling quite comfortable, sitting out in front, then Damir started lifting the rate and I realized he was becoming quite a big threat so I tried to chuck in some strokes, Drysdale said. "He kept closing the gap.

"About 50 metres from the line I realised it was anyone's. I think in the end it literally came down to the surge. I was fortunate enough to have the boat just surging through the line at the right time."

The television feed initially posted Martin as the winner then removed the result. The final time was given as 6 minutes 41.34 seconds. It went down to thousandths of a second, with Drysdale coming out on top.

"We're sitting there and Damir sort of looked at me and said what's the result and I said I had no idea. I'm very happy that I was on the right side of it," the world record holder says.

"I had no idea. I got the feeling he just passed me and I just chucked in a few short ones in desperation. It was not the way you want to finish, but to come away with that result was fantastic."

Martin took the outcome with dignity and did not appear to be overly disappointed.

"I gave 1000 per cent of myself," he says. "I am more satisfied with a good race than a silver medal. The next one is going to be another colour."

"It's one of the perfect ones, where I did 100 per cent, even more. The last 100 metres was incredible, the shouts from the spectators were crazy."

Synek, who has claimed all of the world championship titles since the 2012 Olympics in London, had to settle for the bronze.

It was Drysdale's third medal after his gold in London in 2016, and a bronze in Beijing in 2008, making him the first New Zealand rower to win three in the same event.

Before his triumph in London in 2016, Drysdale had vomited with anxiety. He says he felt good this week but on Saturday before the race, the nerves were there and he nearly threw up again in his pre-race row.

"You feel like your sick but I knew I wasn't. The nerves give you that little bit of adrenaline."

Asked about his future, Drysdale, who is nicknamed Grandpa, noted he would be 41 at the next games in Tokyo.

He will make a decision in the next three or four months and says it is about 50/50 as to whether he will mount another Olympic campaign.

Drysdale's gold, New Zealand's second in Rio after the men's pair of Hamish Bond and Eric Murray won on Thursday (Friday NZT), gave the Kiwi team eight total medals - with six silvers - and moved them to 13th on the medal table.

- Stuff