
A search is underway involving boats and two helicopters are conducting aerial sweeps.
"Emergency services were alerted about 3pm that someone had entered the water to assist another person, and had then failed to exit the water themselves," police said in a statement.

A Hato Hone St John spokeswoman said it sent one ambulance, one first response unit and one helicopter to the scene.
"We have assessed and treated one patient who was transported to Lakes District hospital in a minor condition by road," the spokeswoman said.
An Otago Daily Times reporter on the scene said there were six boats on the water which appeared to be undertaking a search.
A helicopter was on a sandbank near the wharf.
A police car and an ambulance were also at the scene.