Civil defence notices cautioned motorists, boaters and beachgoers to beware of caustic plumes of "laze" formed from two streams of hot lava pouring into the sea after cutting across Highway 137 on the south coast of Hawaii's Big Island late on Saturday and early Sunday.

The bulletins also warned that reports of toxic sulphur dioxide gas being vented from various points around the volcano had tripled, urging residents to "take action necessary to limit further exposure."

Laze - a term combining the words "lava" and haze" - is a mix of hydrochloric acid fumes, steam and fine volcanic glass specks created when erupting lava reacts with sea water, Hawaii County Civil Defence said in a statement.

"Be aware of the laze hazard and stay away from any ocean plume," the agency said, warning that potential hazards include lung damage, as well as eye and skin irritation.

Under Sunday's conditions, with strong winds and copious amounts of lava hitting the ocean, the laze plumes could extend as far as 24 km, according to USGS geologist Janet Babb.

The US Coast Guard was "actively monitoring" the area to keep away all vessel traffic except permitted tour boats, the civil defence office said.

An air quality index for Kona, about 64 km northwest of the eruption site, was at "orange" level, meaning that older individuals and those with lung problems could be affected.

Kilauea, one of the world's most active volcanoes, began extruding red-hot lava and sulphuric acid fumes through newly opened fissures along its eastern flank on May 3.

The lava flows have destroyed dozens of homes and other buildings, ignited bush fires and displaced thousands of residents who were either ordered evacuated or fled voluntarily.

On Saturday, authorities reported the first known serious injury from the eruptions - a homeowner whose leg was shattered by a hot, solid lumb of lava called a "lava bomb" while standing on the third-floor balcony of his home.

Mark Clawson, a friend of the victim who did not want his name used, lives near where his friend was staying as caretaker, and heard screaming and saw the harrowing aftermath, he told Reuters.

Apparently a fiery "lava bomb" about the size of a dinner plate was launched from a fissure about 200 metres from the house, Clawson said.

"Most of them (lava bombs) arc high in the air, but every now and again there's one that gets shot like a rifle, more horizontal and that's what happened here," he said.

He said doctors had to pick sharp, hardened fragments of lava out of the wound, but the prognosis is good for his friend.

Australian Associated Press