Auckland Transport said a train shuttle was operating between Onehunga and Penrose.
Passengers travelling to stations on the Southern Line between Otahuhu and Newmarket should use scheduled buses from Great South Rd, it said.
The delays come on the first day of the electric train roll-out, with Auckland Transport replacing its remaining old diesel trains with electric trains on all lines from Papakura and Swanson to Britomart today.