People drag friends from fiery boat explosion on Hawkesbury River

Security footage of a boat explosion on Sydney's Hawkesbury River has shown how close a group came to death as four people remain in hospital with severe burns.

Eleven people were at a marina off Dangar Road at Brooklyn just after 5pm yesterday when the vessel erupted into flames.

A mother and her child narrowly avoided being caught in the blast, with security footage revealing how quickly an afternoon on the water turned life threatening.

It was after a pit stop to fuel up when the skipper of the 28-foot cruiser turned over the engine that the explosion occurred.

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A nearby cafe turned into a makeshift triage centre, with bystanders running with containers of water or hoses to help those suffering from significant burns until paramedics arrived.

Cain Betterson, the local postman, was one of the first on the scene, using his tinny to spray river water at the blaze.

"I know the old girl puts up a big bit of water so it was the best I could do and get it away from the bowser," Mr Betterson told 9News.

"With the amount of fuel that's on that and the amount of fuel that's here and all the residents … I love all these people here."

Witnesses have recalled number of young children getting off the vessel just moments before it caught fire.

NSW Ambulance said 12 crews were dispatched to the scene along with a specialist medical team, a Toll Ambulance Rescue Helicopter and a Westpac Ambulance Rescue Helicopter.

NSW Ambulance Inspector David Morris said the incident as the worst he's seen in 15 years and described the scene as chaotic.

"We assessed a mother, a two-year-old child on the scene and thankfully they sustained no injuries," he said told 2GB's Ben Fordham.

He confirmed that the woman and a number of children had only just departed the boat to go and buy ice cream minutes before it went up in flames.

"The bystanders did a fantastic job providing water and cooling these patients at the scene before we arrived," he said.

"Currently this morning we still have one critical and three serious patients at Royal North Shore.

"We're still waiting for an update on the other two patients at Concord and Westmead."

It is believed a number of the patients have burns to more than 60 per cent of their bodies.

Of the four in hospital, one man and a woman are in a serious but stable condition in hospital. One woman remains in a critical condition.

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Firefighters were involved in driving ambulances in order to free up paramedics to work on the patients as they were raced to hospital.

NSW Police provided escorts for the ambulances to help them through the heavy Easter weekend traffic.

The blaze was extinguished but the boat was destroyed and sunk in shallow waters.

The cause of the fire is being investigated.