He was flown by the Northland Emergency Services Trust rescue helicopter to hospital.
The other brother died at the scene, police said.
A search by the police search and rescue team will continue today to find their brother-in-law.
At this stage the names and ages of the men involved would not be released, police said.
A relative of one of the drowned men told the Herald on Sunday he arrived at the beach, north of Maunganui Bluff, north-west of Dargaville, about 40 minutes after the alarm was raised.
He discovered a frantic scene, as about 20 relatives mounted a rescue attempt on boogie boards.
"People were running around mad, really," said the witness, who declined to be named.
Some used boogie boards to try to reach the trio, one of whom was still waving for help when the witness arrived. Others climbed steep hills around the beach to act as spotters, he said.
Coastguard and Northland Emergency Rescue Helicopter joined the operation, but were hampered by stormy conditions.
Coastguard Georgie Smith said they were alerted at 1pm by a beach resident who saw the men swept into the ocean.
"Conditions made it incredibly difficult to reach the victims with two of the fisherman drifting into the land and the other later spotted in the breakers by the rescue helicopter.
"Sadly one of the men who drifted ashore is now deceased."
Kaihu Tavern publican Grant Wikaira often collects shellfish from the coastline, but rarely sees people in the water.
"It's a west coast beach, big waves come in. You've just got to be very careful. One minute the water's at your knees, the next it's up over your head."
A resident in Aranga, about 8km from the scene of the drownings, described the beach on the northern side of Maunganui Bluff as "very, very isolated".
There was no road, with the only access over Maori land.
However, the beach was popular with locals, she said.
In August last year, a 14-year-old boy drowned at Maunganui Bluff.
Meanwhile, three young boogie-boarders and their father were rescued after being caught in a rip at Mokau Beach, north of New Plymouth, yesterday.
A 15-year-old male was flown to New Plymouth hospital in a serious condition after the incident.