Data obtained over 10 yr at the Port Hacking 100-m station show an annual cycle in some properties. In May the continental shelf is dominated by high salinity subtropical water from the western Pacific. From June to August, cooling and vertical mixing decrease temperatures and salinities at all depths. From September, tropical water of northern origin penetrates southwards, increasing temperatures and decreasing salinity at 10 and 50 m. This tropical water exerts its maximum influence in March. From September to March other offshore water from deeper than 100 m rises onto the shelf, decreasing temperature, salinity, and oxygen, but increasing nitrate. In addition there is probably a continual land drainage influence on the hydrological regime. Algal blooms occur from August to April, in association with the penetration of nitrate-rich water onto the shelf.