The opening of Sydney's $21.6 billion metro rail line has been postponed indefinitely due to regulatory delays and safety checks.

NSW Transport Minister Jo Haylen confirmed that the government has not secured approval from the national rail safety regulator and does not have a new start date.

The Transport Minister aims for an opening by August 4, pending regulatory approval and safety assurance.

The cancellation of the August 4 start date was influenced by a significant incident on the Metro Northwest line, which required emergency rescues for over 100 commuters.

Haylen noted that the incident complicated the opening plans and required additional assessments by the regulator, extending the timeline.

Several exercises with firefighters need to be completed before final approval can be granted for the Metro City and Southwest line.

Discussions between the Fire Brigade Employees Union and Sydney Metro regarding safety testing and protocols are ongoing, with a recent lifting of a ban on critical safety exercises.

Sydney Metro has completed over 11,000 hours of testing and conducted more than 200 exercises to prepare for the line's operation.

The rail safety regulator is assessing evidence to support the safety case for the metro line before granting accreditation.

Safety and reliability are the top priorities for the government regarding the metro line, according to Minister Haylen.

Opposition leader Mark Speakman labeled the delay as disappointing and embarrassing for the government.