The man, 22, and the 23-year-old woman, both Indian nationals, were at the WaterMarc aquatic centre in Greensborough in Melbourne's north-east when they were found unconscious in the pool about 7.40pm on Sunday.

It is believed they knocked heads while jumping into the water.

The parents of the students are travelling to Australia to be by their hospital bedsides.

Victoria Police said the woman was dragged from the pool by a patron after she was found floating face down and fully clothed. While she was being treated, the young man, who had suffered a cardiac arrest, was found at the bottom of the pool. Paramedics said he was not breathing and had to be revived.

Senior Sergeant Justin Goldsmith said the couple had been in Australia for two years. They lived together in Preston and both were studying IT.

He said there was a small mark on the on the man's forehead but the couple had no other obvious wounds and there was no blood in the pool.

"There's no indication as to what sort of incident took place for this to happen," Senior Sergeant Goldsmith said.

"There's a small abrasion on the man's forehead, it's a possibility there's been a head clash but there may be other avenues we are investigating."

Senior Sergeant Goldsmith said they were not sure why the woman was fully clothed.

“Whether she was wearing clothes for cultural reasons, was just standing on the side and not intending to get in the water, or whether she jumped in to save the other man is pure speculation. We just don’t know, which is why we’re really desperate for witnesses to come forward."

Senior Sergeant Goldsmith said there was a “strong possibility” that one or both of the victims were already in the water but got into trouble.

“The area where they’ve been found was very close from the drop off point from the shallow into the deep water, and it’s quite a significant, sudden drop off,” he said. “There is still a possibility that they clashed heads. We just don’t know.”

There was no suggestion that the couple had been clowning around or behaving irresponsibly before the incident, he said.

He described the accident as "freak" and said there were more than 100 people in the pool at the time. He appealed for witnesses who saw the couple to contact police.

Senior Sergeant Goldsmith said the parents of the injured couple were flying from India to be by their bedsides as they remain in a critical condition at the Austin Hospital.

Earlier, Ambulance Victoria general manager Graeme Parker said it was unclear whether the man and woman clashed heads while diving or afterwards.

Mr Parker said the woman ‘‘had a pulse, but had difficulty breathing because of the amount of water she had ingested’’.

The most difficult part of the rescue, he said, was removing the pair safely from the pool.

‘‘The paramedics had to employ defibrillation equipment in a dry area, and [the victims] were both very wet,’’ he said.

Mr Parker commended pool staff for dragging the pair from the water and performing CPR while safely evacuating the centre, which was ‘‘packed’’.

‘‘The lifeguards had the appropriate equipment and training, and they gave those people every chance of survival by the way they handled the situation,’’ he said.

Mr Parker warned people to be careful around water, especially during a time of heat, when many go to public pools to cope with the hot weather.

‘‘It’s a tragedy – young people going out and enjoying their day at the pool as most of the others were doing, and for some unforeseen reason something so simple has happened and resulted in such a tragedy.

‘‘Although water activity is a great pastime and leisure activity ... it’s time to remind the community that they need to be careful around water,’’ he said.

A Banyule Council spokeswoman said Watermac had adequate staff rostered on at the time of the incident.

She said seven lifeguards and a duty manager were working in the pool hall, which had an estimated 250 swimmers.

Life Saving Victoria guidelines stipulate that there should be 1 lifeguard for every 100 swimmers.

A manager at WaterMarc said staff had successfully handled the situation. She refused to comment further.

WaterMarc is a $42 million Banyule City Council project, and is managed by Belgravia Leisure.

It claims to be the largest aquatic and leisure facility in the northern region of Victoria, and has several indoor pools and waterslides.

Swimmers who attended Watermac on Sunday posted on the aquatic centre’s Facebook page that the pool was overcrowded.

“What a disgrace,” Chris Everest said. “I was there with my family and the amount [of] people that were let in was ridiculous.”

Kim Hill said: “I was there yesterday morning with my two young children, and there appeared to not be enough lifeguards on duty. It was extremely busy, but there surely needs to be a limit on how many swimmers there are at one time in the pool, and more lifeguards on duty."

Jo Lee said it was time to rethink the number of people allowed in the pool at one time. “I looked down from the wellbeing room into the pool around 5pm. There were more people th[a]n pool.”

Kerrie Lynne said she no longer takes her children to Watermarc on weekends or hot days because it was too crowded to watch them safely. “Watermarc do have a duty of care, and numbers should be limited,” she said.

Michelle Garzia branded lifeguards as “incompetent” and claimed her daughter witnessed a previous drowning.

“Not the first time someone has been lying on the bottom of the pool unconscious. My daughter and a friend had to alert lifeguards once. It was a woman in her 30s. This was in the lap pool and there would have only been a handful of people swimming. Incompetent lifeguards.”

Ambulance Victoria attended six other water-related accidents on the weekend, including the deaths of a child and a man.

Paramedics were unable to revive the five-year-old boy, who was unconscious when pulled from the water at Lysterfield Lake Park in Narre Warren about 5pm on Sunday.

Ambulance Victoria spokesman Ray Rowe said two MICA units and a helicopter were sent to the park, where paramedics tried to resuscitate the boy.

Earlier on Sunday, the body of a Victorian man was found after he disappeared in the Murray River. The 27-year-old was last seen swimming under the Echuca-Moama Bridge on Saturday evening.

After an extensive search, the man’s body was found downstream from the bridge about 10am on Sunday.

A NSW police media spokeswoman confirmed the man was a Victorian resident but had no further details.

On Sunday, a young boy was pulled out of the water at Myrtleford, 48 kilometres south-east of Wangaratta in Victoria’s north-east. He was flown to the Royal Children’s Hospital in a critical condition.

Another boy was injured on Sunday after he hit his head on the side of a pool while doing a somersault at Berwick, in Melbourne’s south-east.

Two men were knocked out at the weekend, with one man in his 30s losing consciousness after he hit a submerged object while water skiing at Ebden, in Victoria’s north-east.

Another man, believed to be aged in his 40s, lost consciousness when he hit his head while using a waterslide. He was taken to Western Hospital, Footscray.

A man in his 60s was pulled out of the water at Portsea beach after he had difficulty breathing. He was taken to the Frankston Hospital in a stable condition.

with Beau Donelly