Auckland Council has apologised to visitors whose cars were locked in the Mt Eden domain for two hours after closing and who were asked to pay a $40 release fee.
About 80 people from 20 cars were queued at the gates on Tuesday night when they were caught out by a new, earlier closing time of 8.30pm.
Before Sunday the park was open until 11pm. Several visitors said signage indicating the earlier closure was small and not visible from cars.
As co-managers of the maunga, Auckland Council yesterday apologised but claimed only five cars were at the gate.
It's the second time in a month the council has come under fire for contractors locking cars in a public park at night.
In January Deidre Smith and Deb Brookes and their horses were shut in Sanders Reserve near Paremoremo after they could not pay a $40 cash fee to a guard from Global Security. The women had to stay in the reserve overnight.
One of those trapped at Mt Eden, Vika Dorokhova, said the situation was handled badly and the fee only agitated an already "irate mob".
"Unfortunately, the First Security officers that eventually arrived on the scene refused to release the vehicles unless fines of $40 per vehicle were agreed to be paid."
When no one would comply the gates were opened after 10.30pm, she said.
David Shaw, who was escorting two tourists up the summit, said it took almost half an hour for security to arrive and another hour before they were freed.
First Security is owned by Wilson Group, a company that operates Wilson Parking, which has been criticised this week for issuing $65 parking fines for late returning drivers.
First Security general manager Mike Rutherford said it was standard practice to charge a release fee to anybody locked in a council property.
He said it did not take two hours to resolve and guards used "common sense" when they didn't impose the fees.
Auckland Council governance director Grant Taylor called the incident "regrettable". He said the council was investigating and working with the security contractor to ensure it didn't happen again.
The council's manager of volcanic cones, Scott De Silva, said visitors got plenty of warning from security guards who arrived at 8.30pm to find 70 cars still inside the park.
All left except five cars, he said.
Mr De Silva confirmed signage would be reviewed. Visiting hours to Auckland's volcanic cones were standardised in December. They open at 7am, and close at 8.30pm in summer and 7pm in winter.