Police took an armed man into custody early this morning, after a standoff at a petrol station in the Waikato township of Mercer.

There is no risk to motorists and the Waikato Expressway had been reopened after the man was taken into police custody at 1.35am.

Police late last night closed the Waikato Expressway - disrupting the travel plans of hundreds of motorists, some of whom told the Herald they had seen police with assault rifles at the scene.

In a media release just after 11.30pm police said they responded to a report of a man allegedly in a vehicle with a firearm at Mercer Mobil petrol station.They were first alerted to the vehicle in Manurewa and, with the assistance of the Eagle helicopter, followed it to the petrol station.


"Once the vehicle came to a stop, a female has exited the vehicle and advised police a male in the vehicle had a firearm with him."

Police had cordoned and contained the incident and armed offender squad members from Waikato had been called to help. Motorists were being told to avoid the area.

This morning, the station's manager Manpreet Singh told the Herald that the staffer working at the time of the incident, who was aged "22 or 23-years-old," was "a bit shocked," and would be given a few days off work.

Mr Singh said the station had not been damaged, and police evacuated the staff member soon after the gunman entered the store.

He said the entire incident was caught on CCTV, and he would review the footage in the next few days.

Te Kauwhata woman Cassandra Keenan was returning home from Takanini and was among those diverted away from the scene.

"I saw three police officers carrying assault rifles."

At the Last Post Tavern, about 100 metres from the petrol station and neighbouring McDonald's, about 50 patrons continued to enjoy their evening.

A staff member told the Herald that evacuated Mobil and McDonald's staff were outside, along with dozens of police cars.

"There must be 30 police cars for one d***head. The cops told us to stay put and we've got the McDonald's and Mobil people standing outside. They just said 'there's a nutcase in there with a gun'."