More people living near the Rewa River in Fiji have had to be evacuated to emergency shelters because of flood waters after Cyclone Mick.
The deputy representative for UNICEF in the Pacific, Tim Sutton, says aid officials are concerned that the water may becoming stagnant in hot temperatures.
He says roads east of Suva remain flooded and the airport has had to close during high tide.
He says many more people have been displaced, evacuated to an extra 24 evacuation shelters.
"We're very concerned. Simply of that, the 2,600 in evacuation centres we can estimate that at least 1,500 of those are children under the age of 18. And they are particularly vulnerable as they have just recovered from a severe flood back in January."
Tim Sutton says flooding is of the same scale as it was in January.
New Zealand is making a 70,000 US dollar contribution to relief efforts in Fiji following Cyclone Mick.
The money will be used on essential relief supplies, including the replenishment of supplies used by the Red Cross to assist people in the immediate aftermath of the cyclone.