Blummenfelt broke away late in the closing10km run leg to win from Briton Alex Yee with Kiwi Hayden Wilde taking the bronze, while Jake Birtwhistle was Australia's best performer in 16th.

In embarrassing scenes at the start line, a camera boat blocked the entry of almost half of the 51 competitors into the water at Tokyo's Odaiba Marine Park.

The boat then almost reversed over some swimmers, while support craft frantically tried to get the message of a false start to those in the water to steer them back to the pontoon.

The race was safely restarted about 10 minutes later.

With a little over two kilometres to go, the three medallists cleared away before world No.2 Blummenfelt made his winning break to secure his country's first medal in the sport, winning by 11 seconds with a time of one hour 45 minutes and four seconds.

The 27-year-old Norwegian collapsed shortly after crossing the finish line amid sweltering conditions.

"I'm from Bergen where for two thirds of the year it's windy, rainy and 10 degrees but that just shows the strength of the team around us in getting that preparation," said Blummenfelt.

"I've been thinking about this race for so many years. I was on holiday here many years ago and thought to myself 'OK, your goal is to win an Olympic gold medal here in 2020."

One of the race favourites, Britain's Jonny Brownlee, was bidding for his third successive Olympic medal having won bronze in London and silver in Rio, but had to settle for fifth while French world champion Vincent Luis finished 13th in the sweltering conditions.

Queensland's Matt Hauser was 24th while Aaron Royle, from NSW was two places back in 26th.

Royle and Birtwhistle were right in the hunt midway through the bike leg, sitting second and third behind Luxembourg's Stefan Zachaus.

But they were swamped by the chasing pack and at the final transition Birtwhistle was 12th, 16 seconds off the race lead, while Royle was 24th and Hauser 25th.

Birtwhistle, who won silver at the 2018 Commonwealth Games, was bitterly disappointed he couldn't find something extra in the run leg, finishing one minute 28 seconds behind the winner.

"It was all looking good but I just didn't quite have the legs on the run," the 26-year-old told Seven Network.

"I felt strong on the bike and stayed in pretty good positions and did everything right up until the run but just didn't quite have the legs when it mattered.

"It's going to be probably three years of kicking myself until Paris (Olympics) but we've got another chance on Saturday in the relay - we've got a great team so let's hope Australia can deliver then."

Australian Associated Press