Lifesavers are urging people to follow warnings and stay out of dangerous waters after a woman watched on as her husband and son drowned in surf south-east of Melbourne.

Key points: The men could not be revived after getting into trouble off an unpatrolled beach

The men could not be revived after getting into trouble off an unpatrolled beach A heatwave is forecast for Victoria

A heatwave is forecast for Victoria Lifesavers are warning swimmers to stay between the flags

Emergency services were called when a man and his two sons got into trouble in difficult conditions while swimming off Phillip Island about 5:30pm on Christmas Eve.

Local police Senior Seargent Andy O'Brien said the Cranbourne family of four was at an unpatrolled part of the coastline known as The Colonnades, just along from Cape Woolamai surf beach in Newhaven.

"Three members of the family went into the water … [there were] difficult conditions, they all got into trouble," he said.

"The 12-year-old son was removed from the water fairly quickly and he was OK, but the father and son both got into trouble and they couldn't be rescued."

The 45-year-old father was pulled from the water but could not be revived by paramedics.

The police airwing was called in to help locate his 20-year-old son, who was found in the water by lifesavers from Woolamai Beach Surf Lifesaving Club (SLSC) and died at the scene.

"This poor family will live with this for the rest of their lives — every Christmas it'll be a reminder of what's occurred and that's terrible," Senior Sergeant O'Brien said.

"[The woman] was very upset obviously last night, which is understandable, she had her young son who also had to get some treatment … so her focus was also obviously on ensuring he was OK," he said.

"It's just an absolute tragedy. You can't even begin to think how the people are feeling today."

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He said the drownings had also been upsetting for the first responders and onlookers.

"It's tragic any time, but at this time of the year I think it hits home really strongly with everyone involved," he said.

Lifesaving Victoria state duty officer Kane Treloar said volunteers had done "a tremendous job in some really challenging conditions".

'We'll continue to work across the summer to keep everyone safe and to do our best to make sure everyone who goes to the beach comes home again."

Lifesaving Victoria said their volunteers tried to save the men in challenging conditions. ( ABC News )

The area is known for being dangerous, and lifesavers warned swimmers to watch out for rips after two drownings in a week this time last year.

"It's notorious, that portion of the coastline, for rips and large waves and rough seas," Senior Sergeant O'Brien said.

"It's happened far too often down on this coast over the last few summers, and we're really imploring people to be safe around the water."

The Woolamai Beach SLSC reissued a warning to avoid the area, saying conditions were too dangerous and volunteers were not patrolling.

Lifesavers brace for heatwave

Lifesaving Victoria is preparing for a busy week, with a scorching heatwave forecast across the state beginning on Boxing Day.

"My advice to people as they're heading down the beach over the next couple of days as it's warming up is to check the conditions before you're going, always take someone with you and, where possible, swim between the flags," Mr Treloar said.

The Bureau of Meteorology is forecasting five days over 40C at Mildura, in the state's north-west, when the heatwave hits.

The southern half of the state will be a few degrees cooler, with Melbourne expected to reach the mid-30s by Thursday.

Senior forecaster Beren Bradshaw said Mildura was expected to reach 45 degrees on Friday.

Cape Woolamai after a man drowned on Christmas Day last year. ( ABC News: Kyle Harley, file photo )

"The warm weather does stick around the state across the next few days and well into the weekend," Ms Bradshaw said.

"The northern half of the state will be seeing heatwave conditions with maximum temps over the next few days getting above 40 degrees and into the mid-40s well into the weekend."

Police are continuing to investigate the drownings.