Three people are dead and a fourth is in hospital after a boat flipped at Auckland's Manukau Harbour this afternoon.

Police, the Westpac Rescue Helicopter, Maritime NZ, St John Ambulance, the Coastguard, and local lifesavers all responded at about 4.40pm, after the boat carrying the group overturned near the bar.

Police say all four people were found in the water but only one survived.

The person who survived suffered serious injuries and has been flown to hospital.

Police said they were looking into what caused the tragedy.

It brings the death toll of people who have died in the harbour in a week to five.

Last weekend, two people died in water incidents on the Manukau Harbour in Auckland's first weekend of relaxed Covid alert level restrictions.