In short:
The National Rail Safety Regulator said there are safety checks yet to be completed for Sydney Metro.
NSW Premier Chris Minns said the rail line's opening was on track and the regulator "only comes in at the very end".
What's next?
The major rail project has been scheduled to launch this Sunday.
Five days before the newest instalment of Sydney's metro line is due to open to the public, the national rail safety regulator says several critical safety checks are yet to be completed.
The highly anticipated Chatswood to Sydenham route runs under Sydney Harbour, directly linking north-west commuters to the CBD, and is scheduled to open this Sunday.
NSW Premier Chris Minns insisted the opening was "on track", but acknowledged the paperwork was yet to be completed.
"It was going to be late in the piece because it needed to be," Mr Minns said.
"The regulator only comes in at the very end when testing and all the applications are in place, but as of Monday morning we're on track."
Metro yet to be given tick of approval from safety regulator
Trial runs and testing of the new line have been underway for several months and the fencing around some new stations was pulled down on Monday morning.
However, the Office of the National Rail Safety Regulator (ONRSR) said the metro was yet to be given the tick of approval.
"There are still several critically important activities to be completed this week, such as emergency and evacuation exercises with NSW Fire & Rescue, along with the provision of documentation to support the operational safety case to enable ONRSR to properly assess the application," a spokesperson said in a statement.
"ONRSR will assess the application as quickly as possible once all the evidence is received, noting that it requires sufficient time to undertake the analysis of the proposed operational safety case."
The regulator could not give an estimate on how long such applications would usually take to be approved.
Fire and Rescue NSW (FRNSW) Commissioner Jeremy Fewtrell said his service's part of the assessment was also still "ongoing" and had no firm timeframe.
"It's quite a complex and involved process, that is ultimately overseen by the national [rail] safety regulator," Commissioner Fewtrell said.
"We're going through a whole range of testing processes as part of the commissioning processes with metro and providing our feedback accordingly.
"That work's ongoing … we just keep working through it and getting things done as quickly as possible."
'Code red' lifted last week
Last week, the Fire Brigade Employees Union (FBEU) lifted a "code red" it had in place, barring members from taking part in any "familiarisation and drill exercise[s]" related to the Sydney Metro.
"The FBEU have worked with FRNSW on four robust cross-agency exercises to be conducted on Metro lines," state secretary Leighton Drury said.
"The Code Red has been lifted to allow these exercises to be undertaken."
State Opposition Leader Mark Speakman said while "safety always comes first", any potential delay to the metro rollout would be a "bungle".
"Sydney Metro would never have happened unless it had been planned, designed and funded by the Liberal and Nationals," Mr Speakman said.
"We were a government that delivered transformational infrastructure right across NSW, there is nothing transformational in Chris Minns' pipeline except cutting the ribbons on our projects."
The Chatswood to Sydenham metro line is due to be put to the test of commuters on Monday and is estimated to cut travel times for thousands of people.
Travel between Martin Place and Waterloo should take six minutes, Sydenham to Macquarie University should take 33 minutes, Central to Chatswood should take 15 minutes, and North Sydney's Victoria Cross to Barangaroo is estimated to take three minutes.