Cooper denied gold in bizarre swim final

Australia's Isaac Cooper has been denied a backstroke world short course swimming gold medal in bizarre circumstances after the final had to be re-run.

Cooper was first home in the 50m final in Melbourne on Friday night but less than half the field completed the race after an alarm sounded due to a "technical error".

An hour later the race was re-run with Ryan Murphy touching ahead of Cooper, however the American deemed the 18-year-old Queenslander the true champion.

Murphy's winning time was 22.64, slower than Cooper's initial time of 22.49 seconds which was a junior world record and a personal best.

Cooper clocked 22.73 in his second effort to pick up the silver medal.

"I was pretty disappointed the way that it shook out," veteran Murphy told reporters.

"I really feel for Isaac - he's 18 and going for your first individual world title and that's huge and an incredible accomplishment.

"I'm going to talk to him and let him know that in my mind he won that race."

A shattered Cooper avoided talking to the media.

Cooper, who was banished from the Commonwealth Games team for misusing prescription medication, was bidding for his third medal after winning bronze in the men's 100m backstroke behind Murphy and gold in the 4x50m relay.

In the first race only three swimmers including Cooper continued with no rope falling at the 15m mark to stop the swimmers.

Officials conferred before deciding to reschedule the race with a full field of eight swimmers after they ruled there was no false start.

Dolphins coach Rohan Taylor said there was unanimous agreement from competing countries to re-swim the final.

"It's the only fair way - there was a malfunction of the starting system so there was no other option," Taylor said.

"There were people who didn't swim the race. 

"Obviously Isaac might not be happy with that (winning silver) and I understand that and I totally respect that but it's about what you take away from these thing and learn."

Dual Olympic champion Ariarne Titmus, who is skipping the titles and working in commentary, said she'd never seen such a race unfold.

"There should've been a 15-metre rope that goes down when there is a false start, when someone leaves early so that it stops the boys from swimming the rest of the race," Titmus told the Nine Network. 

"Isaac has been vocal in the fact that after a 50 or 100m effort he can feel quite sick and even vomit. 

"They've definitely got a disadvantage compared to the boys that heard the noise to stop in the race.

"I would be pretty upset - he has just done a PB, technically looked at the scoreboard and thought he's become world champion then he hears no crowd."