Eight protesters have been arrested on an oil drilling support vessel at Timaru's port.
Thirty protesters have been occupying the OMV support vessel at the port for more than 30 hours.
They are trying to stop the Skandi Atlantic from servicing the Austrian oil company's drill rig off the coast of Taranaki.
A police spokeswoman said they've been maintaining a presence at the scene to keep the peace.
But she said they've recently had to remove eight people from the boat - who'd moved into an unsafe area.
It's unclear at this stage whether charges will be laid.
The Bahamas-flagged OMV was preparing to leave the port of Timaru and travel north to the oil rig off the coast of Taranaki, RNZ reports.
Greenpeace said OMV is about to begin drilling three oil wells there.
Police said they were called to the port at about 6am.
Climate and energy campaigner Amanda Larsson said those taking part in the occupation are prepared to stay overnight.
"By occupying OMV's henchboat, we're delaying the monster rig from drilling for new oil in the middle of a climate emergency," she said.
"OMV's oil rig is almost 50 nautical miles off the Taranaki coast. Its remote location means it's nearly impossible for ordinary New Zealanders to take peaceful action to confront it directly.
"We've brought the resistance here to disrupt its chain of support. We won't let this rig carry out its climate-destroying activities quietly, out of sight and mind."
She said representatives from other environmental groups such as Extinction Rebellion, Oil Free Otago and 350 Aotearoa are also involved.