Authorities in Solomon Islands say at least 18 children have died after suffering from extreme diarrhoea.

The Ministry of Health has declared a nationwide alert, saying the virus has also affected more than 1,000 people in six provinces.

Rotavirus has been confirmed as the cause of the outbreaks in Honiara, Guadalcanal and Gizo.

Symptoms, including diarrhoea and vomiting, usually appear between one to three days after being exposed to the virus.

Health workers in Solomon Islands say unsanitary conditions caused by the floods in April have created ideal conditions for the diarrhoea outbreak.

Residents are being urged to wash their hands with soap and use clean towels after using the toilet.

Under Secretary for Policy and Planning Chris Becha says the virus can spread easily.

"Because this is a rotavirus infection, it is highly contagious and spread is through person-to-person contact, contaminated objects, food and drink," he said.

"Rotavirus can be prevented by thorough hand washing for 10 seconds using soap and water and drying with a clean towel after going to the toilet or caring for someone with diarrhoea, and before preparing or eating food."

Dr Becha says the deadly cases involved very young children.

"The verified figure is all of them are under five in the past two weeks.

"And there are 1,000 people who are affected and we urge them to take extra care as far as this outbreak is concerned."

Andrew Catford, country director at World Vision Solomon Islands has told Pacific Beat water supplies were contaminated during the floods.

"There was widespread water inundation to people's property," he said.

"A lot of mud and unsanitary conditions and particularly one of the issues has been the open wells particularly in Honiara, but also Guadalcanal and other places that they are quite easily contaminated."

Australian Foreign Affairs Minister Julie Bishop says Australia's High Commissioner in Honiara is directing resources towards the outbreak.

Ms Bishop says Australia is monitoring the situation and stands ready to further assist Solomon Islands.