Several people have been arrested at a climate protest in Newcastle that involved hundreds of activists taking to the city's harbour in canoes and kayaks.

The demonstration, organised by activism group Rising Tide, was advertised as a "blockade" of the world's largest coal port.

A Rising Tide statement said multiple people were arrested on Saturday afternoon, most of whom were released without charge.

In a statement police said officers with the Marine Area Command removed three people from the water at the Port of Newcastle.

"Two men aged 27 and 60, and a 26 year-old woman, were arrested and taken to Newcastle Police Station where they are assisting police with their inquiries," the statement said.

Local doctor Peter Schofield said he was escorted by police back to land after entering the shipping channel but was allowed to walk free.

"I'm a health professional and I'm very very concerned about climate change and the health consequences of climate change," he said.

"We know what needs to be done … the government's not doing it, so we need to give them a prod."

Midnight Oil frontman and former environment minister Peter Garrett spoke to the crowd and performed a few songs at the event.

"I strongly support the peaceful protest that's planned here," he said.

"Consider what the scientists are telling us, look at the agreements we're trying to forge internationally and understand that this is one of the most defining issues of our time.

Safety concerns

More than 100 people were arrested at last year's Rising Tide protest on Newcastle Harbour.

It was unclear what the event would look like leading up to this week's "protestival" after the NSW Supreme Court sided with an application from the state's police commissioner to reject an application to demonstrate due to safety concerns.

That meant protesters could be charged by police for obstructing vessels.

Rising Tide said it would go ahead with the event because protesters had the right to peaceful assembly on land and water.

Maritime NSW then established an exclusion zone for Newcastle Harbour from Thursday night to Monday morning, but that was deemed invalid by the NSW Supreme Court and revoked.

Shortly after that judgement on Thursday, Premier Chris Minns said he was "disappointed" by the outcome.

"We don't want to stifle protest — we want people to be safe and if they're going to protest they need to do it, we hope, in a safe way," he said.

Police Minister Yasmin Catley, who is also an MP in the Hunter, slammed the action.

"Police believe this is dangerous and fear the worst," she said in a statement.

Out in force

The heavy police presence has not deterred protesters.

Thousands of people have already attended the event, which will continue over the weekend.

Kurubeenun Neal is a Ngarrabullgan man who travelled from Cairns.

"We been fighting for our land since I can remember," he said.

"We're sovereign people — we come here, stand with this mob here, because we're protecting our land, they're protecting our land."

Andrea White travelled from Melbourne. 

"Climate impacts are being felt all over the world, people are being displaced from their homes, species are being driven to extinction," she said.