A group of Kiwis were struggling to free their boat's anchor from rocks when an "enormous boom" alerted them to a volcanic eruption unfolding before their eyes.

Mt Stromboli is dotted with three active caters at its peak and has been in a state of near eruption for centuries, often spitting glowing molten rock into the sky.

But on July 3, it erupted in a blast that killed a hiker and sent a great big plume of smoke towering above the Tyrrhenian Sea.

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Hobbyist film-maker Susan Bellerby, a Raglan resident of 15 years, was boating nearby with her husband Frank and four good friends when the volcano erupted.

"We just heard this enormous boom, explosion.. we looked up and it was just... 'Oh my God'," Bellerby said.

There was lava rocketing up into the sky and fires taking light all down the side of the mountain, she said.

International media reported that some tourists had thrown themselves into the sea, while others jumped into boats to evacuate.

"It was pretty scary actually," Bellerby said.

"Our anchor got stuck in the rocks at the time.

"We called the coastguard for help because we couldn't get the anchor up. We couldn't leave the area and we were a bit nervous about the situation."

But the coastguard could not send anyone to help the Kiwi group because of the emergency unfolding at nearby Stromboli.

That response, amid murmurings of a possible tsunami threat, convinced the group to cut anchor and head off.

By chance they came across a group of Italian freedivers as they were leaving, she said

"They were completely oblivious. They were just out in the open sea."

The divers went back with the group and helped them retrieve the anchor.

Speaking to the Herald from nearby Salina hours after the explosion, Bellerby said the sky was still cloudy with ash.

"You can barely see the volcano now, it's covered with cloud."

Less than an hour before the eruption the Kiwi group, made up of university mates, stopped on the other side on the island and discussed venturing up the volcano, a hike often made by tourists.

Stromboli is one of eight Aeolian Islands that forms a volcanic arc just north of Sicily, Italy. Graphic / NZ Herald
Stromboli is one of eight Aeolian Islands that forms a volcanic arc just north of Sicily, Italy. Graphic / NZ Herald

But they were feeling a bit too hot and bothered to make the climb and instead took the boat to go swimming nearby in an area known for bubbling underwater thermal activity.

"It is just one of those situations where we were just lucky... we just can't get over it actually."