Ponter said KiwiRail has flown in an expert from Auckland this afternoon who will work through the night to try to isolate the problem.
KiwiRail operations executive general manager Paul Ashton said the fault affected track points between Wellington Railway Station and Ngauranga.
"Specialist engineering staff are working to fix the fault as quickly as possible. The inconvenience to Wellington commuters is regretted."
In a statement, Metlink said ongoing meetings have been held throughout the day as rail experts worked to resolve the problem.
Eight railway tracks merge into two at the junction as trains approach and leave loading platforms.
A disruption timetable is in place tonight and will apply until the morning.
Until further notice all services from Upper Hutt and Waikanae are running every 30 minutes, Melling branch services have been temporarily cancelled, peak Taita, Porirua and Plimmerton services are cancelled, and Johnsonville and Wairarapa services are running as normal.
Transdev, the train operator, will be organising buses to depart from key stations across the network.
If the fault is found and remedied before the peak travel period tomorrow, Transdev will work to get extra train services running between Taita and Porirua, Metlink reported.
However, commuters were urged to make alternative travel arrangements where possible to avoid lengthy delays both tonight and tomorrow morning.
Passengers should regularly check the Metlink website for information on current services.
Any changes will be posted as soon as possible.
