The alleged offender was still present. He was taken into custody and had since been interviewed.
The victim was taken to Whangarei Hospital by ambulance, then flown to Auckland City Hospital where he underwent emergency surgery on Saturday and again on Sunday.
Mr Johnston said the man had suffered massive trauma to his brain, requiring the removal of parts of his skull to relieve the pressure and prevent further damage.
He had suffered other injuries - a full medical report was still being prepared - but the head injury was the most serious.
The victim was last night still in a critical condition in the hospital's critical care unit.
Mr Johnston said police were continuing to interview witnesses and family members yesterday as well as completing their scene examination.
It was "extremely fortunate" that family members had the sense to record the incident on their cell phones. Each clip was a couple of minutes long.
"The events are all captured clearly. It's fair to say that these recordings provide excellent evidence and have assisted us greatly in this inquiry."
Mr Johnston would not say whether the men were related or what kind of weapon had been used.
Charges against the 51-year-old could be upgraded if the victim does not recover.
Any other witnesses are urged to contact Kaikohe police on (09) 405 2960.