Most of the NIMT is single track with frequent passing loops, built to the New Zealand rail gauge of . The line is double track between Wellington and Waikanae (except for a single-track section through the tunnels, between North Junction, which is from Wellington and South Junction, which is from Wellington, on the Pukerua Bay to Paekakariki section), between Hamilton and Te Kauwhata (except for the single-track Waikato River Bridge at Ngāruawāhia), and between Meremere and Auckland Britomart. Around (approximately 65%) of the line is electrified in three separate sections: one section at 1600 V DC between Wellington and Waikanae, and two sections at 25 kV AC: between Palmerston North and Te Rapa (Hamilton) and between Papakura and Auckland Britomart.