Up to $40,000 is being spent each day to prevent protesters delaying work on the Roe Highway extension across the Beeliar Wetlands in Perth's south.

The project, known as Roe 8, is part of the state government's $1.9 billion Perth Freight Link project to carry heavy transport to Fremantle.

Despite strong opposition from environmentalists, WA Labor and Indigenous and community groups, work on the controversial project began in early December.

Today it was revealed that over a three-day period this week $120,000 was spent policing the area - with 100 officers stationed there on each of those days.

WA Police Commissioner Karl O'Callaghan said if the same level of police continued to be required, it would become a cost pressure.

"It's an enormous drain on police resources," he said.

"We've had 100 officers there for three days this week every day. So that's 300 police officers all day long managing Roe 8."

On Tuesday protesters chained themselves to a gate and police had to close a road after heavy machinery was employed to begin clearing the land.

Police said at least seven people were arrested on charges including trespassing, obstruction and refusing to give details to police.

Premier Colin Barnett has stood firmly behind the project.

"The project will go ahead and if they continue to protest and interfere they will be arrested and charged," he said on Tuesday.

However, he said, it was chewing up police resources at a cost to taxpayers.

A Main Roads spokesperson said the project would take more than two years to complete.