Australia's busiest harbour at risk of sewage eruption

A leaking underground pipe has prompted fears sewage could flow into Australia's busiest harbour.

Emergency works are underway on a 100-year-old central pipe carrying excrement up to nine metres below Sydney Harbour.

The leak caused by a "significant wastewater infrastructure fault" was identified at a critical rising main beneath the Pier Street on-ramp at Darling Harbour.

A permanent repair could not occur until the main was fully isolated and flows were redirected, Sydney Water said in a statement on Friday.

In the meantime, the leak is being contained and monitored.

Urgent emergency works are underway at the nearby Wentworth Park in Glebe to divert the flow away from the leaking pipe.

NSW Water Minister Rose Jackson said under the "worst-case scenario" leaked sewage could flow into the harbour, though the likelihood was low.

"We are diverting the leaking ... but that's not a permanent solution," she told reporters in Sydney on Friday.

"When we have leaks of that nature ... we don't want to risk a more significant impact particularly at this time of year.

"The reality is when we're talking about significant impacts with sewage, things can get messy."

Ms Jackson said crews were hampered by the age of the infrastructure.

"This is a very old part of our network," she said.

"The area where the leak has happened is over 100 years old.

"This is very, very early infrastructure (and) that does make fixing it and isolating it and the contingencies around it more challenging."

Sydney Water chief executive Darren Cleary said there would be no environmental impacts from the leak.

Permanent repairs to the affected pipe are expected to take several months and will start when flow diversions are fully in place.

Stage one of the diversion works is on track for completion by Christmas, while stage two works in Quarry Street, Ultimo and Mitchell Street, Glebe are planned for early 2026.