Two brothers were injured when an 80-kilogram marlin flipped out of the water and sliced the pair as it thrashed about in their boat on the NSW Mid North Coast.

Three men, including the brothers, were boating in the Solitary Islands Marine Park near Wooli, north of Coffs Harbour, about lunchtime on Thursday, when they hit the marlin, NSW Police said.

The Westpac Rescue Helicopter took one of the brothers to hospital. Credit:Westpac Rescue Helicopter

The boat was travelling about 40km/h when it hit the fish, which the brothers estimated weighed between 80 and 100 kilograms, NSW Police said.

It flipped out of the water and landed in the boat, NSW Police said.