The effects of dredge spoil disposal on contaminant concentrations and benthic macrofauna were examined at a shallow marine disposal site used for c. 20 years. The site had received c. 50 000 m3yr‐1of maintenance dredgings annually, from the Port of Nelson. Port sediments were contaminated to varying degrees with some trace metals, organo‐chlorine pesticides, polychlorinated biphenyls, and polycyclic aromatic hydrocarbons. They showed mildly elevated toxicity in laboratory bioassays, and their macrofauna was dominated by small‐bodied polychaetes. Despite this, there was very little indication of impact in the spoil disposal area. The disposal area, spoil spreading zone and control sites were all similar in terms of sediment contaminants, sediment toxicity, neogastropod imposex, and macrofauna. The lack of discernible impact is probably because of the dynamic sedimentary environment in the disposal area, which disperses dumped dredgings and mixes them with ambient sediment.