Auckland councillors are frustrated they are still in the dark about when the city's new underground commuter train network will open, despite a meeting updating them on its progress.

Auckland Transport will hold public open days for the City Rail Link (CRL) from next month, but is waiting for sign-off from the rail safety regulator, the New Zealand Transport Agency (NZTA).

At a committee meeting on Tuesday, project leaders from Auckland Transport, KiwiRail and CRL updated councillors about the network's progress.

However before discussion began, Auckland Council director of resilience and infrastructure Barry Potter said they could not answer questions about the opening.

"We won't be talking today about the opening date except to say it is getting closer day by day, and hopefully it's not too far away from announcements.

"We'll just leave that, if we can avoid questions around that that would be great."

Councillor Richard Hills said people were getting frustrated knowing it was close with no confirmed date.

"Is there any way in the next couple of days that it can be announced what month? Because there are a lot of people getting upset, concerned. There's a lot of businesses who just want hope."

Soon after, Councillor Daniel Newman tried a backdoor question: "The new timetable, on what date does that apply?"

Auckland's rail network closed earlier this month for a final dress rehearsal for the long anticipated CRL.

Auckland Transport said that went well and formal testing had been completed, with regulatory approval the last step. That would come from NZTA and the director of land transport.

An NZTA spokesperson said a significant volume of documentation and evidence had been submitted, and this was being assessed thoroughly by the regulator.

"For the CRL, three separate safety case variations have been submitted for approval - covering the new rail infrastructure as well as changes to freight operations and passenger services due to the CRL opening.

"This process provides independent assurance that everything 'so far as reasonably practicable' has been, and will continue to be, done to reduce the risk of harm."

Auckland's rail network had a major shut down for a month - from the end of December until 28 January - to test the CRL and allow for final track rebuilding.

The CRL update showed there would be future shutdowns this summer, but these would be shorter and only partial closures so KiwiRail could complete network maintenance.

Deputy Mayor Desley Simpson said it would be crucial to ensure any partial shutdowns did not happen when there were big events in the city.

"I know we're not going there with the opening date, but if you think it's got to be somewhere between now and Christmas, there's going to be a very short period of time when everything opens up, then you begin to do partial shutdowns again."

Auckland Transport group manager for rail services Mark Lambert said the train network would remain open until January 2026, then there would be a series of partial closures that would be well publicised on the southern and western lines. He said there would be two weekends this November when there would also be partial closures.

Councillor Mike Lee said such closures were unacceptable for a modern city.

"I'm not too sure when it will be open, obviously, but if this practice in terms of disrupting the network… is going to carry on, any idea of getting a serious passenger commuter rail service in Auckland to be viable… is not going to happen," he said.

"If this sort of disruption is going to carry on there's going to be a real problem… from the people of Auckland. That is just not acceptable"

CRL open days

The opening date was a tightly held secret, but Auckland Transport had released some information about the lead-up. It had a new journey planner tool that showed the new timetable for when the CRL opens, allowing commuters to check if it will be faster to take a train.

Auckland Transport, City Rail Link Limited and KiwiRail were also opening the new stations to the public through a series of 'First Look' events. These will be free, ticketed sessions, but numbers will be limited.

They will be held over consecutive Saturdays in August: 8 August - First Look at Maungawhau, 15 August - First Look at Karanga-a-Hape, 22 August - First Look at Te Waihorotiu.

At peak, trains will run every five minutes through the city centre and on average around 10 minutes elsewhere on the network. At off-peak the new lines will operate every 15 minutes throughout the day, including weekends, compared with every 20 minutes at present.