A vessel with seven people on it overturned near Stewart Island this afternoon, with two people requiring medical assistance.

A Police spokesman said the vessel overturned at Freshwater Bay, and police received the report of the accident at 1.45pm this afternoon.

Inspector Mike Bowman said all seven people were rescued and transported to Half Moon Bay. 

"All people were treated for various levels of hypothermia and two are being transported to Southland Hospital for further assessment," Mr Bowman said.

"Police are very thankful for the quick response provided by the emergency service volunteers and the wider Stewart Island community."

Police are assisting Maritime New Zealand with enquiries to investigate the incident.

Police could not confirm whether it was a commercial or recreational vessel.