In February last year, councillors approved an estimated project cost of $19.4 million, which included construction, materials and project management.

However, three extra programs that were "not part of the original scope of works" have added an estimated $2.86 million to the project, the council said.

As part of the unplanned projects, NBN fibre-optic conduits were laid through the mall and Telstra replaced about 70 telecommunications pits, most of which contained asbestos.

The council also negotiated with Sydney Water to replace the 80-year-old high-pressure water mains running down Crown Street. The council plans to approach the water service for contributions towards the extra costs once works are complete.

Lord Mayor Gordon Bradbery defended the extra costs, saying the council was given no choice but to work with Telstra, NBN Co and Sydney Water.

"These were things we unfortunately could not plan for . . . but we needed to get them done properly, because if the water main had failed down the track, it would have ruined the brand new mall," he said.

"At the same time, Telstra discovered that its pits contained asbestos, so they had to be replaced . . . and then there was the NBN, which we wanted to put in so all the businesses could access high-speed internet."

Cr Bradbery was not sure the council would recover costs from Sydney Water, although negotiations were continuing.

"Under the rules of engagement in this sort of stuff, it's the person who lifts the surface who has to usually wear the blame and the cost," he said.