A state of emergency has been declared on the Vanuatu island of Ambae, as the volcano that towers from the island's centre continued to erupt, forcing thousands to flee their villages.

The volcano - known as Monaro - has been rumbling for weeks, but its activity increased rapidly on Saturday, when it started belching ash across much of the island, blanketing villages and crops in the north and south.

Photo: Vanuatu Disaster Management Office

That prompted authorities to evacuate half the island's population - at least 5000 people - from the north and south of the island, sending them to the east and west.

"It's quite a serious emergency," said Manuel Amu, the chairman of the island's disaster committee. "Moving people from their communities into an area with very limited houses and very limited resources like food and water. It's really a challenging issue.

"At the moment the volcano has blown up ash and dark smoke, with a little bit of lava," he said in an interview.

New Zealand scientists were monitoring the volcanic activity with an Air Force Orion due to fly over Vanuatu today.

GNS vulcanologist Steve Sherburn told Morning Report recent photographs showed it could have a devastating impact on the island.

"There's now a cone in the lake that's completely isolated from the water. So it means the eruptions will be dry and therefore will produce a lot more ash.

"And this is where we are going to get the impact on people living downwind and [their] agriculture."

The country's Geohazards Department on Saturday raised its alert for Ambae from level three to four, which is classified as a "moderate eruption state." According to a department spokesperson, the volcano's activity had not changed on Monday, with no increase nor decrease in the intensity of the eruption.

Photo: Vanuatu Disaster Management Office

However, earlier in the weekend authorities swung into action to relocate thousands of people from the remote villages scattered across the rugged, bush-covered 400 square kilometre island, which sits between Santo and Pentecost about halfway up the Vanuatu archipelago.

Several ships had been deployed around the coast to pick up evacuees - as many as 1000 people on one journey, said Mr Amu - and cars and trucks packed to the hilt made their way across the island's narrow, winding and bumpy tracks to safety.

Peter Korisa, the operations manager at the National Disaster Management Office in Port Vila, said authorities were taking action now in case the eruption intensified.

"The major hazard at the moment is to do with ash fall and acid rain. We're using whatever resource we have, some local vessels, commercial vessels, as well as the local transport," he said. "We want to be more proactive before we have to worry about pyroclast (where rocks and other fragments are hurled by the eruption) and stuff like that."

Mr Korisa said evacuation was compulsory and police officers were being used to help convince people to move.

Photo: Vanuatu Geohazards Department

About 15 evacuation centres had been opened in the east and west, mostly in schools and community halls, Mr Amu said. However, assistance from the national government would be needed to deal with such large displacement.

"We've got enough accommodation. But at the moment we have to carry out an assessment to consider other issues relating to sick people, the pregnant women and children and people with special needs. We also have to address the issue of food security, wash, and probably education for those schools which have been closed," said Mr Amu.

One option, Mr Amu said, was to relocate the vulnerable to neighbouring islands - including Pentecost and Maewo - to help ease the burden on Ambae, which he said already lacked the resources to cope.

"The provincial government has been for the last two weeks trying to support these people," he said. "We put our request for the national government to come in if possible to support us with food and water and other necessities.

"At the moment we need help from the national level and if necessary then we need overseas friends to help us with the situation we are facing."

Photo: Vanuatu Geohazards Department

According to the government, the state of emergency it declared on Monday would immediately release funding and national resources for such support.

Most of Ambae live on subsistence crops, which were being pelted with ash and acid rain. Additionally, most peoples' water supply came from rainwater tanks, which were now also contaminated. Mr Amu said while only half the island was being evacuated, the whole island would need assistance.

Further west, residents on the country's largest island, Santo, said crops risked being ruined by ash if the wind direction did not change.

The last time there was a significant eruption on Ambae was in 2005, when a similar evacuation of 5000 people was carried out. It was as long as three months before people could return to villages and there were many complaints of evacuees being contained in squalid conditions.

Photo: Vanuatu Geohazards Observatory file

Mr Korisa admitted conditions for evacuees would likely be basic again this time around. "There is an issue with basic services," he said. "We're talking about islands that don't have electricity and the water system is a big challenge for us. In 2005 there was the same incident and it's the same situation again we're trying to deal with. We're hoping to have learned from that."

"Hopefully this time most of the challenges, we hope those challenges won't occur again. That's why we called in the national director of disaster to intervene very quickly during the weekend," said Mr Amu.

He said authorities were also preparing for the possibility that the eruption could get worse. If the alert level was raised from four to five, the entire island may have to be evacuated, and authorities were preparing for such a situation while hoping it would never come to fruition.

"Most people are not coping very well," Mr Amu said. "We're feeling the pressures."