Security staff raised the alarm around 5:15 a.m (1915 GMT Monday) after protesters climbed the landmark bridge.
"The security on the bridge did respond very quickly but with the volume of people that were there... they prevented quick access".
Six Greenpeace activists scaled the bridge this morning to demand that Prime Minister Scott Morrison takes action on climate change.
Earlier in the operation ten people were arrested.
They were charged with risk safety of another by climbing structure, wilfully prevent free passage of person/vessel, climb on any part of a bridge, enter enclosed land without lawful excuse. A short time ago, a further three protesters who were attached to ropes beneath the bridge were arrested.
Thirteen people have been taken to police stations to assist with inquiries.
No charges have been laid yet.
Climate change activists have scaled Sydney Harbour Bridge, with at least three reportedly arrested.
The protest began around dawn, with a number of people involved.
Police are at the scene directly above the three protestors.
The Greenpeace supporters unfurled banners calling for action as they abseiled from positions underneath a public walkway on the western side of the bridge.
A group of indigenous Torres Strait islanders lodged a formal complaint against the Australian government at the United Nations on Monday, claiming that their climate inaction violated their basic human rights.
Morrison said Australia would meet its commitment to the 2015 Paris climate accord but enacting stronger targets would damage its finances.
Greenpeace Australia Pacific's chief executive, David Ritter, dismissed questions about the legality of the protest action.
"Coal is the number one driver of climate change in Australia - we urgently need a plan to phase out coal power to be replaced by clean energy", he said.