Water sports have been cancelled and fishermen warned to stay off the entire Hauraki Gulf this weekend as authorities wait for the results of tests on dead dogs and marine life.
Initial tests that could shed light on the deaths of dogs, dolphins, pilchards and penguins are expected back on Monday and Tuesday.
In the meantime, health officials have issued a warning against any water contact - fishing, swimming, diving, eating seafood, or taking dogs or children to gulf beaches over the weekend.
Despite a forecast for a dry and mostly sunny weekend, the New Zealand Dragon Boat Association had to cancel a planned training event for between 60 and 100 people, while kayak courses and multi-sport training on the gulf were also moved or cancelled.
North Shore City Council spokeswoman Kelly Gunn said normally popular beaches such as Takapuna were virtually empty yesterday afternoon, despite unseasonably warm weather.
In other developments, specialist algae scientists from the Nelson-based Cawthron Institute found samples of potentially toxic "mat" algae at Cheltenham beach.
The evidence "strongly suggested" algae was the culprit, said Dr Susie Wood, a freshwater scientist at the institute.
If so, it would be a world first, as marine algae was known only to kill birds, sealions and other marine animals, while freshwater algae was known to kill dogs, she said.
Brown-black mat algae clings to rocks and seaweed, can be washed on to the sand and produces an earthy smell attractive to dogs, she said.
Unlike a colourful, water-borne algal bloom, it could go unnoticed on the beach.
Dogs that ate even a small amount of toxic algae could froth at the mouth, become paralysed and die.
Government authorities are treating the dog deaths as separate from mass deaths of pilchards in the Gulf. Dr Wood said mat algae would not explain fish or penguin deaths.
Climate scientist Jim Salinger analysed wind trajectories from July 18 and found strong westerlies and sea surface temperatures which he said suggested deep water was mixing with the sea's surface - possibly pushing up nutrient-rich water that could promote algal growth.
That would not explain any earlier possible algal growth, he said.
Dr Wood said toxic algae liked warm water, high in nutrients.
Despite the warnings, many people plan to get out and enjoy the water this weekend.
Organisers said Auckland Rowing Club's planned 7km race for about 100 adults and high school pupils would go ahead tomorrow on Tamaki Estuary. They would warn people of the risk beforehand and let individual rowers decide.
Phil Scott of Adventure Fishing Charters said he and his wife had eaten fish from the Gulf every day this week and suffered no ill-effects. Reports of marine deaths were causing panic and he was getting calls from people concerned about going out fishing.
Bruce Newbury, of Dr Hook Fishing Adventures, said he saw 50 or 60 seemingly healthy dolphins just beyond the harbour yesterday. He planned to take a fishing group out today and did not expect any problems. "From what I've seen in the media it is pilchards that are dying [and] we use imported pilchards for our bait."
ARC chairman Mike Lee suggested there could be a link between the deaths of sea creatures and a die-off of cockles and starfish near Leigh in January and February.