A serologic survey of anti-Brucella and antileptospiral antibodies was conducted on 147 adult, female New Zealand sea lions (Phocarctos hookeri). Most sea lions (n=138) were sampled at Sandy Bay, Enderby Island, Auckland Islands (50°30′S, 166°17′E), January 2000–March 2005. Nine were sampled at Otago, New Zealand (46°0′S, 170°40′E); four in April 2008 and five in March 2009. Serum from one of the Enderby Island females was weakly positive for antibodies to Brucella abortus using the competitive enzyme-linked immunosorbent assay, and one female had a low titer for Leptospira interrogans serovar pomona using the microscope agglutination test. All serum samples from Otago animals were negative. Brucellosis and leptospirosis are therefore considered unlikely to play a major role in population dynamics of these populations, and the low antibody prevalence of these agents suggests that they are an unlikely source of infection for humans, wildlife, or domestic species on mainland New Zealand.