"I doubt whether they would have been able to get him ashore under those conditions,'' Mr Turnbull said.
"By the time we got out there it was getting quite imperative that we fished him out before he ended up on the rocks.''
Rescuers were able to winch him up and take him to a waiting ambulance.
Mr Turnbull said he told his rescuers he had to let his brother go.
The brother was cold, but otherwise uninjured, Mr Turnbull said.
Rescue helicopter winch operator Phil Davis-Goff said the surviving brother told him he had tried to rescue Mr Openshaw ``but had lost him''.
They found Mr Openshaw face down in the water, about 500m from shore. His body was recovered and flown back to Whangarei.
Mr Turnbull said the weather was "severe''.
- additional reporting Northern Advocate