A teenage girl and her father were clutching onto an overturned catamaran while three other passengers including the girl’s grandparents drowned in rough seas off Newcastle.

Authorities were alerted when an emergency beacon was activated from the capsized boat 13 nautical miles off Stockton Beach about 10.10am on Thursday.

A 16-year-old girl and her 50-year-old dad were winched from the water and taken to John Hunter Hospital to be treated for hyperthermia.

The girl’s 78-year-old grandmother and grandfather and a third unidentified body were retrieved from the water by Marine Rescue, NSW police said.

“The two people that they retrieved from the water were actually clutching on to the overturned vessel,” Ambulance NSW inspector Luke Wiseman told reporters in Newcastle.

The rescue mission was complicated by high seas, strong winds and debris in the water, he said.

Authorities say all five people were wearing life jackets.

The 11.7-metre catamaran had left Shoal Bay earlier on Thursday morning and was heading south toward Ettalong Beach on the Central Coast.

Detective acting superintendent Grant Healey said it was unclear how the boat capsized, with police yet to speak to the two survivors.

“The conditions were difficult,” he told reporters in Sydney.

“We had 25 to 30 knot westerly winds with the sea at one to two metres coming the other way. So it was fairly messy out there.”

Authorities have not yet been able to recover the overturned catamaran, which is drifting out to sea.

“As soon as there is favourable sea conditions there will be an attempt to salvage the vessel,” Healy said.

“But while the weather is like it is, all we can do is let the boating community know where it is.”

A strong wind warning is in place along the NSW coast, from the Hunter down to Eden.

Roads and Maritime Services are urging fishers and boaties to be extra cautious, with choppy conditions expected on exposed bays as well as dangerously rough conditions offshore.