The discovery of 400 disused utilities, including underground pipes and cables, will push out work on the light rail network in Sydney's CBD by up to five months.
The mostly redundant utilities were found during works on George Street and authorities said they needed to identify the companies that own them before work could progress.
"For every redundant utility find we have to go through a two- to four-week process to validate it, because if we snip it, something really bad could happen and we could take a telecommunication cable or something," said Transport for NSW CBD coordinator general Marg Prendergast.
The utilities are in two zones, including between Market Street and King Street, which is a key shopping area.
Work was meant to finish before Christmas, but was now unlikely to finish until April 2017.
"That has meant that we will still be working in the city at Christmas," Ms Prendergast said.
"In zone five, between Market and King [streets], which is our retail precinct, there will be only low-level non-invasive work that's not as noisy.
"We will activate and really support business in the precinct.
"We will work with them to activate and promote that Sydney is still the place to come and shop and party at Christmas time."
Work on zone six, between Market Street and Park Street, is now forecast to finish in May 2017.
Sydney Light Rail
The CBD and South East Light Rail is a new light rail network for Sydney, currently under construction. The 12km route will feature 19 stops, extending from Circular Quay along George Street to Central Station, through Surry Hills to Moore Park, then to Kensington and Kingsford via Anzac Parade and Randwick via Alison Road and High Street.
