Scientists say there is strong evidence that dogs that died after visiting eastern beaches in Auckland had eaten a toxic algae.
Health officials are warning people not to swim, gather shellfish or allow children on the beach as they try to find out what has killed fish, birds and two dogs.
Cawthron Institute's technical manager Paul McNabb says tests show the presence of an algae called formidum, the same species which has caused dog deaths in rivers.
Mr McNabb says the speed of the animals' deaths is strong evidence the cause was a neuro toxin that quickly stopped their vital orgins working.
Test results are expected on Tuesday afternoon.