Aid workers in Solomon Islands say the outbreak of disease in the tropical heat among the victims of Monday's devastating tsunami and earthquake is a major threat.
Officials say at least 5,000 people have been made homeless by the magnitude 8.0 earthquake and tidal wave.
One doctor at a makeshift hospital in Gizo has appealed for more aid.
Dr George Jalini says conditions are difficult and it is almost impossible to prevent infections in the open-air environment.
Nancy Jolo of the Solomons Red Cross says children in some of its campsites are starting to experience diarrhoea.
She says the biggest needs right now are for water.
World Vision Australia's chief executive, Tim Costello, says the water shortage critical as gravity-fed wells were contaminated with salt water while food supplies had been lost.
The United Nations says there are fears that malaria could spread easily among people who fled their homes after coastal villages were destroyed.
The World Health Organisation has dispatched an expert from Fiji to help with malaria control efforts.
The fear of a disease outbreak comes as the Solomon Islands government warns thousands of people that help may not not reach them until tomorrow.
The charity Save the Children says its personnel have not yet been able to reach some of the most devastated areas.
Its Australia's development general manager, Karen Hill, explains.
"Water supply is the major issue, tanks have been destroyed, water supplies have been destroyed so we need to look at bringing water in but also water purification systems and tablets and of course as the water deteriates then you have a major disease issue possibly on your hands."
Karen Hill says the aid team hopes to reach Gizo later this morning.
Some islands are still out of contact and dozens of villages unreachable by road and officials have declared Western Province and Choiseul disaster areas.
A United Nations disaster response team is heading to the Solomon Islands to help coordinate relief efforts.
A UN spokeswoman says a six-member UN disaster assessment and coordination team has been deployed to the Pacific island.
Taiwan has donated 200,000 US dollars to a relief fund to help rescue and reconstruction.
And the French government has also made available a military CASA aircraft to fly 2.5 tonnes of food supplies and emergency equipment from New Caledonia.