Australians hoping to cool off in the sea over the festive period have been urged to stay safe after a spate of drownings.

Four people died in just two days off the coast and with the hot weather set to continue, authorities have called on swimmers to avoid taking risks.

There have been 30 drownings in 26 days across Australia, most of which were in coastal waters, Nigel Taylor, chief executive of Life Saving Victoria, said.

A 20-year-old man and another in his 40s drowned while swimming at Phillip Island beach in Victoria on Monday, while a young boy with the group managed to swim to the shore.

On Christmas Day, a diver died after getting into difficulties off the coast of Cape Schanck on the state’s Mornington Peninsula.

A 46-year-old South Korean man also drowned on Tuesday afternoon while snorkelling at Lake Conjola on the NSW South Coast.

Mr Taylor urged beachgoers to swim in patrolled areas, check for dangerous conditions and skip the dip in the sea if they feel at all uncomfortable.

“It’s going to be hot, people are going to want to swim,” he told reporters on Wednesday.

“We just suggest to you that you make sure you plan your day before you actually finish up at that particular water environment.”

He warned against twilight swims because lifesavers are not on duty and those looking to cool off in inland waterways should get as much information as possible about their destination, because they are often not patrolled at all.

“We suggest you pick up on local knowledge. If you’re going down for a coffee, it’s a latte and ‘do you know where the best swimming conditions are or do you know where it should be that I shouldn’t swim in this local area?’,” Mr Taylor said.

“If you’re boating, and we have to repeat this message time and time again, everybody has a lifejacket and everybody is wearing a lifejacket.”

Victorian Premier Daniel Andrews called on those heading to the beach to “be smart”.

“Don’t have what should be such a special time for you and your family turn into a tragedy,” he said on Wednesday.

“Be aware of the circumstances, don’t drink and swim and if you are not confident in your swimming ability, then the only place you can swim is at a supervised beach.”

Extreme heatwave conditions are forecast across much of southern Australia throughout the coming days, and could last until the New Year for parts of the country.

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

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.

Cape Woolamai after a man drowned on Christmas Day last year. Photo: ABC News/ Kyle Harley

-with AAP