Auckland's busiest train station will remain closed until Friday morning, KiwiRail says.

Auckland Transport announced this morning that a track fault had occurred near Britomart train station and alerted public transport users at 5.45am to the disruptions.

This afternoon, KiwiRail chief operating officer Todd Moyle said the station will only reopen tomorrow morning.

"KiwiRail staff have been on site all morning, and work to repair the track faults is underway," Moyle said.

"We anticipate the line into Britomart will remain closed today but will reopen tomorrow morning. Further work will be undertaken Friday evening, to fully complete the job."

The track fault was in relation to "extensive" work being done to repair about 100 kilometres of degraded rail across the Auckland network.

"KiwiRail was already aware of the faults near Britomart and had planned to deal with them during a future temporary closure," Moyle said.

"However, routine monitoring of the site last night revealed the Rolling Contact Fatigue – cracks caused by pressure on the rail from trains passing over – was significant enough that they needed to be fixed immediately."

He apologised to public transport users affected by the disruptions and gave the assurance that his team was working around the clock.

"No maintenance work has higher priority than this project in Auckland," Moyle said.

"We thank commuters and neighbours for their patience as this urgent work continues."

All train services that head to Britomart would stop at Newmarket and replacement services are operating between the two stations.

"The Onehunga line is cancelled between Britomart and Penrose," Auckland Transport said.

"Rail replacement buses are running between Newmarket and Britomart."