He understood the driver of the vehicle fled the ramp in a boat before police caught him.
A friend of the victim said: "Boat ramps can get a bit testy at times but I think there's more behind this than somebody hogging the ramp".
He described his mate as "an extremely experienced boatie".
"He's not one who doesn't understand ramp etiquette or anything like that," the friend said.
The area is one of the best whitebaiting spots in the Waikato.
The ramp at the end of Elbow Rd provides access to Waikato river islands including Motutieke and the Kaiwakas.
An Auckland Westpac Rescue Helicopter spokesman said they responded to the incident about 5.35pm on Monday.
The victim had substantial leg trauma and was transported to Middlemore hospital in a critical condition.
"He was in an awful lot of pain ... It appeared the injuries were predominantly to one of his legs," the spokesman said.
Last night, a Middlemore Hospital spokeswoman said the man was now in a stable condition in the hospital's intensive care unit.
Police Northern Communications Shift Inspector Andy Brill said the ambulance staff were called to the incident on Elbow Rd in Aka Aka, just after 5pm.
They had received reports that a car had hit a pedestrian, but on further investigation, police became aware that the incident may have been intentional, Mr Brill said.
