Trains will be replaced by buses on the Southern Line between Newmarket and Penrose for four weeks from next Monday, 21 September.
KiwiRail says it is so it can continue an urgent upgrade of the Auckland metro railway network.
The move follows a four-week closure of the Eastern line between Ōtāhuhu and Britomart, which will reopen to trains from Monday.
Testing shows that more than 100km of rail in the Auckland network needs repairing or replacing.
A speed restriction of 40km/h is in place for all trains across Auckland for safety reasons and a 20 minute timetable has been adopted by Auckland Transport.
KiwiRail chief operating officer Todd Moyle said in a statement that they were grateful for the public's patience while the work is underway.
"Our focus is to get this project completed as quickly as possible to minimise overall disruption for passengers," he said.
More than 17km of new rail and more than 19,000 new sleepers will have been laid on the Eastern Line by the end of this week, he said.
Moyle said KiwiRail and Auckland Transport have agreed on a programme of four-week line closures across the network to get the work done.
"We are currently developing a programme that will primarily focus on working progressively along the Southern Line over the next three months, before moving to the Western line," he said.