A sacculinid rhizocephalan parasite was recently rediscovered in New Zealand after original recordings from collections in 1931. This new species of Heterosaccus occurred in 93.5% of 1118 Metacarcinus novaezelandiae (Decapoda: Cancridae) collected from Wellington Harbour between May 2006, and March 2007. Of the parasitized crabs, 22.1% had a rhizocephalan externa, 62.7% had a scar, and 15.2% had only the interna. Prevalence of Heterosaccus sp. was similar for male and female crabs and all were castrated. Many of the infected males displayed major morphological changes, which included a wider pleon and extra pairs of biramous pleopods compared to uninfected males (feminisation). Newly emergent externa and externa filled with eggs or embryos were found throughout the year, which indicates a year-round reproductive cycle. However, virgin externae were more common at the beginning of the southern hemisphere summer. A literature survey of Rhizocephala in the South Pacific revealed overall few described species (n  =  20), records of nine genera, and several reported but unidentified species in new hosts. The South Pacific clearly has a rich rhizocephalan fauna, but little is known about the ecology of these parasites in this part of the world.