Once in the patrol car the man refused to get out so officers could search him before taking him to the station.
The Independent Police Conduct Authority (IPCA) found while trying to remove him, one of the officers used "unnecessary and disproportionate force".
"At the time the officer struck the man, both of his wrists were being held by officers, he was leaning away from them, and no weapon had been seen," said IPCA Chair, Judge Colin Doherty.
"He was not posing an immediate risk to the officers."
The IPCA said the officer later failed to complete a report about the use of force, which is a requirement.
It also found a separate officer did not use a handcuffing technique correctly, which left the man with ongoing shoulder pain.
That officer was also found to have deliberately placed his knee on the side of the man's head to control him.
The IPCA said this was not a restraint technique that should be used.
When the man arrived at the station he was found to have 708 micrograms of alcohol per litre of breath.
He was charged with driving with a breath alcohol level over 400 micrograms per litre of breath and resisting arrest, later pleading guilty to both.
Police have acknowledged the findings.
In a statement, Waikato district commander Superintendent Bruce Bird said the officers were confronted with an aggressive and highly intoxicated person.
He said they had the best intentions when dealing with the matter.
"Our staff come to work every day to keep our community safe and, in any situation, the use of force is an absolute last resort," Bird said.
He said police accepted a report should have been submitted about the use of force, and this had been addressed via an employment process.
The officer who used a handcuffing technique incorrectly had completed further training.
