Wellington train services will be disrupted at peak times today due to urgent mechanical work.
A large number of train services will not be running and people are expected to have to switch to buses, which might cause long delays.
RNZ understands 20 trains have been pulled from service because of worries about the reliability of their braking systems.
On the Johnsonville line, a number of services have been cancelled or have reduced seating and a half hourly timetable is operating, Metlink said.
No trains will be running along the Melling line, and the service will be replaced with a shuttle bus running between Petone and Melling.
The number of carriages travelling on the Hutt Valley and Kapiti lines is being reduced.
In a statement, Transdev, operators of the Wellington trains, said the urgent mechanical work involves an inspection of some of the carriages.
"The removal of these carriages from service is purely precautionary as there has been no incident where the safety of passengers has been in question. Safety is paramount to us and we are working to return these carriages to service as quickly as possible," the statement said.
The urgent mechanical inspections will affect both morning and afternoon peak times today.