Two people have died and another two are in hospital in a stable condition after a boat got into difficulty in Bluff Harbour this afternoon.
Police said they were notified around 12.20pm that a boat was in trouble in the harbour.
A rescue operation involving police, Coastguard and ambulance staff was launched immediately between Bluff Port and Tiwai Wharf.
A St John spokesperson said two people in a critical condition were taken to Southland Hospital.
Bluff Community Board chairman Raymond Fife said he understood the boat involved was being used recreationally.
"It was rough conditions out on the harbour today. That may have played a part."
He didn't know if those involved were from the area.
"It's tragic, and we feel for the family [of those who died]."
A rāhui has been put in place for the Bluff inner harbour, a post on the NZ Southland Hunting And Fishing Facebook page said tonight.
"As a sign of respect and aroha to those who tragically lost their lives today, also our moana, Te Rūnanga o Awarua have placed a rāhui on the Bluff inner harbour.
"A traditional rāhui requires you refrain from taking kaimoana and pay respect within the notified area."
The week-long rāhui was in place in the area a direct line from Kanawera-Stirling Point to the Papakaha-Tiwai Point, and all the way up the harbour, including Awarua Bay, the post said.
A reporter at the scene during the incident said a helicopter could be seen hovering over the port.
Maritime New Zealand has been advised and police are making inquiries into the deaths on behalf of the Coroner.
Coastguard spokesman Ben Parsons said volunteers from Bluff and Riverton responded to the incident and their thoughts are with the whānau and friends of those affected by the tragedy.
Police would like to hear from anyone who saw a small boat in Bluff Harbour today or who has CCTV footage of the harbour as this may be relevant to their inquiries.
Anyone with information is asked to call 105 and quote event number P048675341.