The introduced, mytilid bivalveMusculista senhousia(Asian date mussel) occurs on the east coast of New Zealand in the Auckland region. Eighteen sites were searched within the Tamaki Estuary: six had extensive mats of mussels and three contained small, isolated clumps. Core samples were taken monthly during 1994/95 from two of the mat‐forming populations. Densities reached 16 000 m−2at Bucklands Beach and 5000 m−2at Farm Cove. Both populations were dominated by a single cohort of mussels. Mussels grew to about 20 mm in 12 months, after which growth virtually ceased. Recruitment was sporadic into existing mats, but occurred adjacent to the monitored mat at Bucklands Beach in April 1995. The area occupied by the initial mussel bed at this site decreased by 60% over 1 year. Further core sampling revealed significantly fewer macrofaunal invertebrates under mussel mats compared to control samples taken from areas of beach without mussels. Infaunal bivalves were most adversely affected byM. senhousia, showing an 8‐fold decrease in abundance within mats compared to cores in the control area. Our results reveal thatM. senhousiain the Auckland area has similar life history features to those reported from populations outside New Zealand. We suggest that any adverse environmental effects caused byM. senhousiaare likely to be local and short‐lived.