Sydney Metro excavator struck CBD gas main

Dominica Sanda and Sarah McPhee
Streets around Martin Place in Sydney have been declared a no go zone because of a gas leak

Authorities are hopeful Sydney's CBD will return to normal more than 24 hours after a high-pressure gas main in Sydney's CBD was ruptured by an excavator working on the Sydney Metro rail project.

A 200-metre exclusion zone established by Fire and Rescue NSW on Friday night around the street-level gas main is expected to start dropping down from midnight when repair works are set to finish.

"Unless there are any unforeseen things, it's expected to be wrapped up by midnight with the majority of the exclusion zone to start dropping down from then," Fire and Rescue NSW acting Inspector Greg Purvis told AAP on Saturday.

"We expect the city will be pretty close to being back to normal by the morning."

Jemena, the gas company with carriage of the pipe, has a team of 20 specialists working on the issue and plans to have the site repaired and returned to normal by midnight.

It planned to temporarily shut off gas near Martin Place about 7pm, reducing the size of the exclusion zone, while making it safe to start repair work from 9pm.

Castlereagh Street, Elizabeth Street and Phillip Street have been closed between Hunter and King Streets since the leak began about 10.30pm on Friday, while Macquarie Street has reopened.

Martin Place train station has also been closed and as of Saturday night would be closed "until further notice," NSW Transport said in a tweet.

Transport Minister Andrew Constance on Saturday admitted the "very serious" incident occurred during preliminary works on the Sydney Metro site.

"An excavator in the area has hit a high-pressure gas pipeline and as a result we've had a major incident," he told reporters.

The minister said he was not "going to get into the ins and the outs" of which company was responsible.

The leak emitted a strong smell and deafening hissing sound, and a number of commercial buildings were evacuated with police and Hazmat crews brought in to assist.

The excavator involved was operated by a third-party contractor with Sydney Metro project director Tim Parker saying high-pressure gas lines require "special sub-contractors".

"For Jemena, there is a spotter so you have someone in the field who's looking over what's happening so if anything occurs on the site, you can see," Mr Parker said alongside Mr Constance.

"We had all that in place. But notwithstanding that, unfortunately, this incident occurred."

He labelled the incident "regrettable" and said they are working with SafeWork NSW to find out what happened.

Trains will continue to run on the Eastern Suburbs line but won't stop at Martin Place, and local buses are being diverted. The City Circle train line is not running this weekend because of trackwork.

Motorists and commuters are being asked to stay away from the area.

The gas pressure was reduced "to a safe level" earlier on Saturday and no customers are without gas due to service diversions, Jemena said.