It comes as activist group Fireproof Australia follow through on their vow to continue demonstrating, despite increased police powers.
NSW Police confirmed a police operation was underway on the Sydney Harbour Bridge on Wednesday, and urged drivers to avoid the area.
The protesters blocked lanes on the Cahill Expressway near the south pylon, over Circular Quay and The Rocks.
The group of four blocked traffic by parking a truck in the left lane, with two people climbing on the cabin, while another two glued themselves to the bridge, a spokeswoman for Fireproof Australia told AAP.
One of the protesters, Alan, has been a firefighter in the Hunter Valley Rural Fire Service for 40 years, she said.
All four protesters are being arrested.
It comes after another member of Fireproof Australia ran onto the pitch with a lit flare during an NRL match between the Cronulla Sharks and the Wests Tigers on Sunday.
The 32-year-old pleaded guilty to two charges of entering enclosed land without lawful excuse and was sentenced to three months in prison for possessing a bright light distress signal in a public place, in Sutherland Local Court on Monday.
Fireproof Australia is calling for the rehoming of flood and bushfire survivors, and the implementation of recommendations from the royal commission into the Black Summer bushfires.
The group has recently held multiple high-profile protests targeted at disrupting traffic and the movement of infrastructure, attracting the ire of the NSW government.
The government recently introduced laws which could lead to protesters facing fines up to $22,000 and up to two years in prison.
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