And he says calling in the army would be an extreme situation, as police remain convinced that de-escalation is the best strategy.
"We've got a good border around this thing - we're very focused on shrinking the protests so that we return the city to Wellingtonians," Coster told Newstalk ZB's Mike Hosking this morning.
The protest against anti-mandate Covid-19 measures is in its 16th day and has taken a nasty turn, with human waste and a "stinging substance" thrown at officers in recent days, with three needing to go to hospital.
A man was arrested yesterday after allegedly driving a car at police.
There are rising tensions, with police actions ramping up and reports of infighting among groups present. A large crowd remains at the scene.
Police visibility would again be increased in the area to provide reassurance to Wellingtonians as they travel to work, school or home.
However, nearby school Wellington Girls' College students will study from home until at least Monday, and Wellington Free Ambulance will no longer attend emergencies within the protest area, both due to safety concerns, RNZ reported today.
Coster said some protest leaders were giving police support, but struggling to effect change.
"There is a genuine conversation going with a group of protest leaders. I'm really concerned about the university and the impact on students coming back.
"I'm concerned about public transport, and we're looking for them to shift the dial on those areas."
However, he said a group within the mob were still causing issues.
"The whole situation is meaning that those influencers are able to cause havoc around the place so the whole protest is contributing to that."
He said police "remained of the view that de escalation is the only strategy that we'll see come out of this in good shape."
"And that needs to be a combination of police taking action when it's appropriate," he said.
Calling in the Defence Force would be a "very extreme situation".
"I don't think any of us wants to see soldiers on our streets and it's unlikely to land in a good place for us as a country, particularly in the longer term.
"So police [are] well placed to deal with this and I just want to acknowledge the outstanding work that our people are doing out there every day."
Hundreds of staff were involved in the response, some returning from other duties.
"A number of those staff wore protective wear, following protesters' appalling actions in throwing human waste at police officers."
The road blocks were installed on Monday to restrict further growth of vehicles in the protest area, and to maintain access for residents, businesses and emergency vehicles.
"We will continue to have staff at each of the road closure locations to restrict further access of vehicles. Police reported cars, trucks and vans continued to vacate the protest area.
"As we have previously stated, genuine protesters are no longer in control of the behaviour in and around Parliament. Police do not wish to interfere with lawful protest, but the behaviours we are seeing are unlawful and will result in enforcement action."
- NZ Herald and ODT Online
