At Bombay, SH 1 becomes the Waikato Expressway, a four-lane dual-carriageway expressway. The expressway takes the highway down the Bombay Hills to Mercer, where SH 1 meets the Waikato River, which it broadly follows for the next . The Waikato Expressway ends in north-western Hamilton. The highway bypasses the city centre to the west, before crossing to the east side and proceeding south-east out of the city. The expressway resumes at Tamahere, bypassing Cambridge to the north before reverting to a single carriageway east of the town. The highway continues eastward to the town of Tīrau, where it turns south to pass through Putāruru and Tokoroa and the surrounding exotic pine plantation forest area.