Tests have confirmed that toxin-producing blue-green algae is floating in the water at Bayfront Park, Pier 4 Beach and the shoreline of Hamilton harbour, public health says.

Staffers have found that blue-green algae is present along most of the western shoreline; from the inner Pier 8 area, through the marinas, and to the closed waterfront trail.

"The public [is] urged to not enter and avoid contact with the water along the western shoreline of Hamilton Harbour," the city said in a news release. "Contact with the water while launching or trailering watercraft should be avoided."

The city says that anyone who comes into contact with the water should wash themselves off as quickly as they can with clean water. Pets should also be kept out of the water.

Health problems are largely caused by drinking water that is contaminated with blue-green algae toxins, but your skin can also become irritated just from touching it.

"Exposure to blue-green algae toxins, if present in sufficient amounts, can cause headaches, fever, diarrhea, abdominal pain, nausea or vomiting, skin rashes and mucous membrane irritation," the city says.

Anyone who has recently been in the waters along the western shore of the harbour and is experiencing any of those symptoms should see a doctor, health officials say.