Heritage advocates say the planned position of portals (tunnels) through the historic Lennox Bridge will desecrate its heritage value.
Last month Parramatta Council approved the tender to build portals through Lennox Bridge to allow better access for pedestrians and cyclists along the river foreshore.
Last year the plan was approved by the NSW Heritage Council despite expert government advice the portals would "seriously and irrevocably compromise" the state heritage-listed item.
Greater Western Sydney Heritage Action Group spokeswoman June Bullivant said the current position of the portals would destroy the heritage value of the bridge.
"We have never been against the portals we just want them to be put through the false abutments that aren't part of the original bridge," Ms Bullivant said.
Their proposal would see the portals built slightly wider from the bridge arch than the current proposal.
"This bridge was built by convicts — once they remove the old masonry it can't be put back," Ms Bullivant said.
The historic bridge was designed by David Lennox and built between 1836 and 1839 — it is considered to be one of the oldest in the country.
The eastern side of the bridge is original while the western side was widened in the 1930s to accommodate the tram.
It is now concrete with veneer sandstone.
Councillor Andrew Wilson, who voted against the construction of the portals, said they could have been positioned in sympathy with the bridge.
"I can't understand how [David] Lennox can build this beautiful bridge and then in the '30s they can do something in sympathy with the bridge and in 2013 we just cut big holes through it," Cr Wilson said.
A Parramatta Council spokeswoman said the council had engaged with heritage consultants and structural engineers for advice on how to maintain the bridge's heritage value.
"During the design and development application process, the portals were moved to be significantly further away from the arch," she said.
"As it currently stands, the south portal is located mostly behind the retaining wall of the old David Jones/Brandsmart building."
The spokeswoman said the designs show the portals to be well clear of the bridge arch.
Ms Bullivant said they would continue to fight against the current positioning.