They also saw some cars doing burn-outs and some disorder, the report said.
"Concerned that there was public disorder as well as a danger to members of the public, a decision was made to temporarily close Maces Road to control the scene and check the vehicles."
Drivers and passengers were instructed by loud hailer to get into their car and stay inside otherwise they would be arrested.
After the vehicles were lined up, NZTA staff inspected the vehicles while police checked driver details, the report said.
The process took about seven hours to complete, and the road remained closed until about 2am.
The authority received 31 complaints from people who were at the scene that evening.
Common issues from those complaints included being detained and the length of time people were detained; the lack of access to toilet facilities, food and water; police videoing people without consent and police being dressed in riot gear and their attitude during the operation.
"The authority has found that given the significant number of vehicles and people present, observed burn-outs and risk of injury to the public from vehicles being driven dangerously, the initial decision by police to temporarily close Maces Road was reasonable and logical," Sir David said.
"However, Police had no power to instruct drivers and passengers to get into their cars and warn them they would be arrested if they got out. This action was unlawful.
"The authority also found that the manner of treatment by police to those unlawfully detained by depriving them access to basic necessities was disrespectful and degrading. It did not comply with police's obligation to treat people with humanity and respect and accordingly breached their human rights."
Police should also not have video recorded the drivers and passengers, Sir David said.
The authority said police had already made some changes to practice and policy following this incident.
However, policy in relation to photographing people is very general in nature and restricted to road blocks.
"Accordingly, the authority recommends that police review their policy in relation to photographing or recording people and provide more detailed guidance to police staff on this issue," Sir David said.
Superintendent Andy McGregor. Photo / Geoff Sloan
Acting District Commander, Superintendent Andy McGregor, said the IPCA report made it clear that police had to balance varying expectations from different sections of the community when dealing with these types of incidents.
"On one hand the community expects police to uphold the law and prevent public disorder, while on the other hand car enthusiasts have the right to hold their events," he said.
"It is apparent that the number of cars encountered at the location was much higher than anticipated, and despite the best of intentions officers were unable to check the vehicles and drivers for compliance within a reasonable time frame.
"As a result of this incident, a thorough review of policies and procedures has occurred. I am confident that similar issues would not arise in any such operation today."
Police accepted the findings of the report, Mr McGregor said.
He said several unsafe vehicles were written off the road, one vehicle was impounded and one driver failed an evidential breath test, due to the checks conducted during the operation.
Since this incident, Canterbury police had run several operations targeting anti-social road users under the new procedures, without any repeat of the same concerns, he said.
