This paper provides an overview of the Sydney Metro Northwest (formerly the North West Rail Link) project and the underground station excavation retention design and construction works, including the key requirements set out in the scope of works and technical criteria (SWTC). Based on the assessment of the geological conditions a soldier piled wall retention system was adopted for all five new underground stations and one of the two services facility shafts for ease and speed of construction. During the Castle Hill Station excavation a new planar wedge instability mechanism was considered to be credible based on the additional geological data, with the original three-dimensional block instability being no longer suitable. This led to redesign of the south wall stabilisation works based on the updated geological model and input parameters. The instrumentation and monitoring plan was also adjusted to ensure the required additional support provided would be adequate for the safety of the station box excavation. The monitored lateral movements at the capping beam and at the inclinometers were within the trigger values, indicating that the retention system constructed was robust.