Activists have scaled the Commonwealth Bank of Australia's Sydney headquarters to protest coal financing.

Two climbers unfurled a sign saying 'CommBank CAN Break Free from Coal' between two CommBank buildings on Monday morning, before being brought down by NSW Police Rescue and taken away in police wagons.

Dozens of protesters packed the base of the building in Darling Harbour, intending to  deliver letters to bank CEO Ian Narev.

The protestors are demanding the CBA start pouring money in clean energy instead of supporting coal.

The climbers, Zoe Buckley Lennox and Kelly Purnell, said they wanted to highlight the impact of coal projects on the Great Barrier Reef, according to a Greenpeace spokesperson.