Boxing legend Jeff Fenech undergoes marathon heart surgery in Thailand

Australian boxing legend Jeff Fenech has successfully undergone lifesaving surgery overnight to replace an infected heart valve.

The three-time world title winner was admitted to a Bangkok hospital three days ago when he began coughing up blood during a training session.

In a five-hour operation, during which Fenech sat up rather than laid down, surgeons replaced the heart valve and drained fluid from his lungs.

The champion boxer's spirits were bolstered by the arrival of his three adult children – daughters Kayla, Jessica and son Beau - in Thailand.

They met with their father just 10 minutes before he was wheeled into the operating theatre and briefly after his surgery.

"It all went fine. He's woken up and he's gone back to sleep," Kayla Fenech told The Today Show.

"I was just crying, but I said I love you and everything's going to be fine, stay strong.

"He's a fighter - he's always been tough. I believe he will get through it."

She also said her father was buoyed by their appearance.

"We literally rushed to the hospital… He was okay, I guess he was scared… He was glad to see Jess and I."

She said her father's ordeal has been tough for all his family and friends. He was disappointed he'd miss Jessica's wedding but he but he had been overwhelmed by public support.

"We can't thank everyone enough for all their kind thoughts and words. He's really happy with all the support he's receiving."

Family friend Dominic Jarvis said Fenech had shown all the tough qualities he displayed in the boxing ring during his health battle.

"The next seven days are going to be incredibly important for Jeff, particularly with the infection status.

"We're hoping he's got more strength left in him – and I'm sure he has – to get through this last hospital aspect of the ordeal."

He also told 9News Fenech's family had set him a goal of walking 75 steps over the 48 hours after his operation.

"His immediate reaction to that was, 'give me one day'," Mr Jarvis said.

Fenech had been in the country with a number of his young boxers, including prodigious talent Brock Jarvis and new recruit Jack Brubaker.

He was rushed to hospital on Friday after days of coughing up blood after his team insisted on calling an ambulance.

They have revealed how close the stubborn boxing legend initially refused to be admitted to hospital.

"I rang him up and he wasn't going to let me in the room. Then one of us realised we had a spare key and we had to go up there and say: 'Jeff, we rang an ambulance,'" trainer Basil Nassis told 9News.

Due to the high risk of further infection, doctors will now monitor Fenech closely following the surgery. He remains sedated and will come out of that in his own time, they say.

His recovery period is expected to take three or four days.

A large group of family, friends and well wishers have gathered to meet him.

Dubbed "the Marrickville mauler", Fenech's decorated boxing career saw him become a world champion at bantamweight (53.5kg), super bantamweight (55kg) and featherweight (57kg).

He represented Australia at the 1984 Summer Olympics in Los Angeles where he was controversially stripped of a decision-win against Yugoslavian Redzep Redzepovski, and won a gold medal at the Oceanian Championships in the same year.

His professional record included 29 wins, 21 of which were by knockout.

Fenech now trains a number of young professional fighters breaking their way onto the world stage.