Twenty-three people have been injured after a volcanic explosion caused molten rock to barrel through the roof of a tour boat in Hawaii.

A woman in her 20s was taken to Honolulu in a serious condition with a broken thigh bone.

The other 22 were treated for minor burns and scrapes, including 12 who were treated at a hospital in Hilo.

Hawaii volcanoLava flows into the sea near Pahoa, Hawaii (Jae C. Hong/AP)

Tour boat operators say they plan to continue taking visitors to see the lava, but will follow the Coast Guard’s revised policy and stay further away.

The Coast Guard prohibits vessels from getting closer than 300 metres from where Kilauea volcano’s lava oozes into the sea.

The agency had been allowing experienced boat operators to apply for a special licence to get closer, up to 50 metres, but it stopped allowing those exceptions on Monday.

Hawaii volcano tour boatAn explosion caused by lava oozing into the sea sent molten rock crashing through the roof of the boat (John Burnett/Hawaii Tribune-Herald via AP)

There are strict limits over access to the lava on land for safety reasons, making boat and helicopter tours the only options people have to witness the volcanic spectacle in person.

Shane Turpin, the owner and captain of the vessel that was hit, said he never saw the explosion.

He and his tour group had been in the area for about 20 minutes making passes of the sea entry about 500 metres offshore, Turpin said.

He did not observe “any major explosions,” so he navigated his vessel closer, to about 228 metres from the lava.

“As we were exiting the zone, all of a sudden everything around us exploded,” he said. “It was everywhere.”

The US Geological Survey says explosions of varying sizes occur whenever  lava enters much colder seawater.

Kilauea is sending as much as 26 times the amount of lava per second to the sea than it did during the 2016-17 eruption.

The Kilauea volcano has been erupting continuously for the past 35 years.

In May its eruption entered a new phase when it began spurting lava through newly formed fissures in a residential neighbourhood.

It has destroyed more than 700 homes since then.