A heroic West Coast surfer is nursing bruised shoulders after spending nearly two hours paddling through wild seas in a "super-human effort" to save two fishermen.

Julian Young, 48, went to Carters Beach at the mouth of the Buller River, near Westport, to check out big surf created by Cyclone Lusi on Saturday.

"I could hear very, very distressed cries. I couldn't see where it was coming from."

Finally, he spotted two men about 400 metres offshore.

"One guy was going under and I knew from past experience he was on his way out."

The pair, aged 59 and 23, had been thrown into the water 10 or 20 minutes earlier when their 2.5m fishing boat capsized and sank while crossing the notorious bar to head up the river.

Young flashed his headlights so the men knew they had been spotted.

He deciding against calling 111 because "these guys needed me right now", so called his wife. With no reply, he rang his neighbour from Carters Beach, fellow surfer Alan Cockfield, to get help.

Young asked two locals to guide him as he plunged into the rough sea.

"I knew the tide was going out and I didn't know if I could rescue them all the way back to the beach."

It took about 10 minutes of vigorous paddling on his surfboard to reach them.

When he got close, they started "hooting with elation".

"The young guy had only about 10 per cent of his energy left. He couldn't swim."

The 23-year-old had a lifejacket tucked under his arm for buoyancy, having taken it off because it kept forcing his head under the water, Young said.

Each man clung to a large orange gumboot, trying to keep them filled with air to float. The older man had no lifejacket.

Young got them to lie across his surfboard while he swam beside them, reassuring them of rescue.

After about 45 minutes, he was tiring and the two men were becoming anxious.

They had drifted 2km offshore with no signs of rescue.

"I just thought I'm going to try to save all three of us with a super-human effort."

He sternly told the men to "get into survival mode".

Young mounted his surfboard to paddle them to safety, the two men kicking to propel them. "I paddled for another hour after that."

Cockfield met them halfway back to shore and took the older man onto his surfboard.

Young was exhausted when they reached North Beach, east of the rivermouth, but no-one was there to greet them.

Unfortunately, debris from the sunken boat had turned up near the rivermouth, tricking rescuers into believing others were in the water.

Soon after, police, ambulance, the rescue helicopter, fishing boats and the Westport surf rescue boat arrived to help. The men were checked at Buller Health and discharged.