solomon floods
© Rachel Skeates/AFP/Getty ImagesPeople search through the debris on the beach near the Solomon Islands capital of Honiara
Forty people are missing and 49,000 homeless after entire communities washed away following heavy rains

Devastating flash floods in the Solomon Islands have killed at least 19 people, while 40 are still missing and an estimated 49,000 people are homeless.

Entire riverside communities and bridges were washed away when the Matanikau river in Honiara broke its banks on Thursday. The government declared a state of emergency.

Rivers in the north-west, central and north of the island also flooded, destroying homes and displacing communities.There are more than 5,500 people in three of the most populous of the 13 evacuation shelters in Honiara, where aid groups report dengue fever is threatening to spread.

Water supplies and infrastructure including sewerage and electricity have also been damaged, Reuters reported.

"This is unprecedented, and I've seen earthquakes and tsunamis and other very bad flooding incidents," said the country director of Oxfam, Katie Greenwood. "But this flash flooding is unlike anything that I've seen previously here in the country."

The low pressure weather system which brought the heavy rains to the Solomon Islands has now become tropical cyclone Ita. Ita, a category one cyclone, is south of Papua New Guinea and travelling westward towards Queensland. The Australian Bureau of Meteorology expects it to intensify to a category three storm over the next few days.

Australia has boosted aid to the Solomons, donating an extra $250,000 in emergency relief assistance.

The floods cut roads throughout the capital, but the Honiara international airport has reopened to limited commercial flights,according to the UN.

The Australian foreign minister, Julie Bishop, said the money would be in addition to an initial announcement of $50,000 on Friday.

Australia will also deploy two engineers and two rapid-response team members to help the Solomon Islands government, UN and non-governmental organisations respond to the disaster.

Bishop has spoken to the Solomon Islands prime minister, Gordon Lilo, and expressed condolences for the loss of life. "[Lilo] said that this is what he wanted - he wanted engineers and relief workers," Bishop told ABC's Insiders on Sunday.

"They didn't want money at this stage because they want actual equipment and people. But we stand ready to support the Solomon Islands. Australia's a very close friend of the Solomon Islands and we'll certainly do what we can to support them at this time."

The Australian high commission in Honiara is contacting Australians in the country to check on their welfare and safety, and is helping recovery efforts along with the Australian federal police officers based there. So far no Australians have been reported missing.

The Department of Foreign Affairs and Trade has changed travel advice after the floods.

New Zealand's prime minister, John Key, said on Monday the country's aid contribution would likely increase "considerably" as further damage assessments are carried out. New Zealand has so far committed $300,000 in aid and supplies.

On Saturday the Solomon Islands was jolted by a magnitude 6.0 earthquake which caused further damage. The islands are part of the Pacific's "ring of fire" tectonic activity zone.