Swimmers are being advised to take caution while dog owners should keep dogs on leashes or avoid the Ruamāhanga River as toxic algae, including detached algal mats, have been identified at the site.
The river site is located near the Mel Parkinson Reserve, north of Masterton.
Greater Wellington Marine and Freshwater Team Leader Dr Evan Harrison said "toxic algae has increased to amber alert levels at the site meaning we strongly advise caution to swimmers and to check the LAWA website regularly for updates before getting in the water".
Harrison said the warning bears particular significance for dog owners as dogs like the smell and taste of the algae and a small piece, only around the size of a 50c coin, is enough to kill a dog.
"The risk is higher still with detached mats being found, meaning the algae has broken off rocks in the riverbed and can wash up on the river's edge, within easy reach for dogs," said Dr Harrison.
With warm weather forecast to continue over the next week, he advised people take caution along with their pets until the warning has been removed from the LAWA website.
In a statement, a Greater Wellington spokesperson said they will be monitoring popular swimming spots around the region on a weekly basis and update the community on when and where it's safe to swim.
People who think they or their dogs have been in contact with toxic algae should see a doctor or call Healthline on 0800 611 116 and for their dogs, take it immediately to the nearest vet.