A 34-year-old woman has died in hospital after being rescued from a rip off Victoria's Phillip Island.

Key points: Woman dies in hospital after dramatic rescue from rip

Woman dies in hospital after dramatic rescue from rip A man remains in a critical condition, another three people are stable

A man remains in a critical condition, another three people are stable Swimmers told paramedics they were in knee-deep water when sand gave way

Swimmers told paramedics they were in knee-deep water when sand gave way Off-duty lifeguards, surfers praised for complex rescue

She was one of a group of work colleagues who got into trouble while knee-deep in the surf near the Cape Woolamai Surf Club about 7:15pm on Sunday.

A 27-year-old man remains in the Alfred Hospital in a critical condition.

Another two men were taken to a local hospital in a serious condition, with one later flown to the Royal Melbourne for further treatment.

A fifth person, a woman aged in her 20s, was taken to the hospital at Wonthaggi after becoming unwell during Sunday evening.

The group of 18 work colleagues were on a day trip to the beach from Melbourne and were taking selfies when they were swept out to sea, according to witnesses.

Paramedic Anthea Chester said the surf lifesaving club was unmanned at the time, but off-duty lifesavers came to their aid.

"There were two surfers, or I'm not sure if they were surfers or surf lifesavers that were off-duty, that just happened to be swimming," she said.

"Those two people successfully pulled all seven people out of the water and started CPR, paged some other surf lifesavers, rang triple-0."

The others who were rescued did not need to be taken to hospital.

Ms Chester said the swimmers told her they were only in knee-deep water, with waves reaching their hips.

"All of a sudden the ground just went from underneath them and they were getting pulled out backwards just out of the blue," she said.

"They all said that as it was knee-deep water they felt completely safe and it was totally unexpected. All of a sudden the ground wasn't there."

Greg Scott from Lifesaving Victoria said it was a complex rescue for the off-duty lifesavers.

"Having multiple unconscious patients at once in the water that you need to manage and multiple resuscitation victims - we're immensely proud of their efforts that everyone left the beach with a chance of survival," he said.

Lifesavers warn beachgoers of dangerous conditions Woolamai beach. ( ABC News: Sarah Farnsworth )

"Had the off-duty lifeguards and surfers not been there then we'd be talking about a very, very different outcome to what we have seen."

Ms Chester said emergency services arrived to a chaotic scene on the beach.

"They'd been pulled up in separate areas of the beach, so there were three or four groups of people all trying to wave to us and call us over at the same time," she said.

"We had to get there and work out which of the patients were more in need so we could go to them first."

Warning ahead of hot weather

People are expected to flock to the state's beaches this week, with temperatures forecast to reach the mid to high 30s.

Mr Scott said beachgoers should swim at patrolled beaches and avoid the water after lifesavers go off-duty.

"After 6:00pm if you're not highly familiar with surf beaches and rips, currents and how they work and how you use them, that's the time to go and do something else," he said.

"Stay on the beach or head off home, but we'd like to see everyone heading home from the beach safely."

He said it was not uncommon for swimmers to get caught in rips, even in shallow water.

"It's not all that uncommon especially with the strength of the rip currents that you can see along the coastline," he said.

"Particularly over summer we'd ask people to be aware of those rip currents.

"Rip currents are deceptive, they do look safe and they do look calm if people aren't familiar with surf beaches and open waterways."