An 11-hour protest against felling heritage trees in Whangarei to make way for a State Highway 1 upgrade has cost roading authorities thousands of dollars.

The New Zealand Transport Agency had planned to cut down 11 trees at the edge of Mander Park and Western Hills Drive on Saturday, but local protesters put a halt to work until today.

NZTA regional director Stephen Town said police arrested three protesters who had been occupying trees overnight.

"Those protesters came peacefully away last night, and this morning we had more security staff on so that we could undertake the work," he told APNZ.

The protesters were not charged and police were understood to have issued them with warnings.

But the delay has added thousands of dollars to the cost of the project, which would involve intersection and road-widening work.

"We'll just attempt to absorb that in the overall cost of the project," Mr Town said.

Mr Town said he was not disappointed and similar protests happened from time to time.

"Small groups of protesters want to make their voice heard right down to the wire. But it will increase the costs of what we're doing over the weekend, but in the scheme of things it's not a disaster.

"The trees are being removed safely and no one has hopefully been hurt or permanently upset by what we're doing, and we will move on."

The local council and NZTA had agreed to an improvement programme at the park, Mr Town said.

"So these emotions die down, hopefully people will see the progress is okay."

Contractors had felled all but four trees by 2pm today, with the rest expected to be cut down by late afternoon.

But the protest action meant the site would not be cleared this weekend, with the stumps to be removed during the week.

Mr Town said fences around the site would be moved off the road by the end of work today and there should not be any inconvenience to traffic tomorrow morning.

Detours were in place over the weekend as the work took place, angering some nearby residents.

Police refused to comment on the protest until tomorrow.

Western Hills Drive was used by up to 25,000 vehicles a day and the roading work would improve traffic flows, authorities said.

- APNZ