The planned opening of a $21.6 billion metro rail line which promises to reshape Sydney and dozens of its suburbs has been shelved, with no new start date identified.

The line was due to open to the public on Sunday but a failure to secure the green light from the national rail safety regulator has forced a delay to the much-hyped start date.

The government will not nominate a new opening date until all approvals are received, which may take weeks rather than days.

More than a decade in the making, the main section of the Metro City and Southwest line between Chatswood and Sydenham comprises six new underground stations, including at North Sydney, Barangaroo and Martin Place, as well as new platforms at Central and Sydenham.

The Herald last week reported the planned opening was coming down to the wire because the National Rail Safety Regulator was yet to complete a number of documents and activities.

Members of the Fire Brigade Employees Union are also still in discussions with Sydney Metro and the Labor government over final testing and other details. The union last week lifted an indefinite ban on members taking part in critical safety exercises but the work is still ongoing.

Sydney Metro chief executive Peter Regan last week said the agency had been clear from the outset that the August 4 start date was subject to final sign-off by the regulator and the operator completing trial running.

The new metro line will take passengers from Central to the new Victoria Cross station in North Sydney in nine minutes, North Ryde to Martin Place in 18 minutes, and Waterloo to the Gadigal station beneath Pitt and Castlereagh streets in four minutes.

Driverless trains will run every four minutes in both directions for three hours in the morning and four hours in the evening peaks, and initially every seven minutes in the inter-peak. The inter-peak frequency will eventually improve to every five minutes in both directions between 10am and 3pm.

More to come