Health officials in the Wellington region are warning swimmers to keep out of the Waipoua River because of potentially toxic algal mats.
Despite a wet summer so far, river flows have been low, allowing the algae to spread; in some places the blue and green mats have come loose from rocks and washed onto banks.
The alga can be fatal to dogs, and contact with people can cause rashes and skin and eye irritation.
A medical officer of health, Dr Stephen Palmer, says algal levels have increased quickly in the river. He says on 5 January there was 6% cover, which has now risen to 36%.
The Waipoua River runs through Wairarapa, including the town of Masterton.