It was one of two boat explosions on Sunday.

A 17-year-old Melbourne boy suffered the worst injuries and was flown to The Alfred in Melbourne with significant burns.

He and his 17-year-old girlfriend were on a speedboat on Lake Buffalo when it exploded after he started it at midday.

“They sustained fairly significant burns, mostly to the legs,” Leading Sen-Constable Brendan Achammer, of Myrtleford police, said.

He praised a man reading a book on the other side of the lake who heard the explosion and jumped on his jet-ski to help.

He created waves big enough to put the flames out and push the boat back to the shore where off-duty nurses helped the injured couple.

Sen-Constable Achammer said the Melbourne family, who housed their boat locally, had checked and started the boat prior to taking it out.

Police were at loss to explain why the explosion occurred.

But initial investigations into a second boat explosion on Lake Mulwala just half an hour earlier indicate petrol vapour as the cause.

Sen-Constable Steve Thorpe, of Mulwala police, said a Wagga mother, father and their two teenage boys were on a boat about a metre from shore near the Yarrawonga Rowing Club.

“They’ve had a bit of vapour under the cover catch fire when the father has started it,” Sen-Constable Thorpe said.

“The boat was totally destroyed.”

The mother and sons were taken to Wangaratta hospital with minor burns and the father was uninjured.

Sen-Constable Thorpe said boat users needed to be aware how easy it was for fuel vapours to become trapped.

“They need to be mindful of that and have proper ventilation,” he said.

Meanwhile, Roads and Maritime Services are investigating a collision between a 14-year-old Melbourne boy and a jet-ski on Lake Mulwala last Wednesday.

The teenager came off his jet-ski and a passing jet-ski hit him.

He was taken to Wangaratta hospital but his injuries were not life-threatening.