Neospora caninum causes disease of the muscle and nervous systems in canine pups and is known for producing disease in older dogs. We aimed to determine anti-N. caninum seroprevalence in healthy adult dogs from greater Sydney, New South Wales, Australia and summarise published reports of adult-onset of canine neosporosis. Indirect fluorescent antibody tests (IFAT) were used to detect anti-N. caninum IgG and compared to the value from an equivalent 1997 study. The N. caninum seroprevalence (anti-IgG, 1:50 serum dilution) was 14.1% (27/192; 95% CI: 9.1–19.0%) demonstrating no significant change compared to 1997 (12%; 18/150; 95% CI: 7.7–18.2%) (P = 0.58). A literature search identified 56 published cases of adult-onset canine neosporosis, most associated with neurological or myopathic signs (66.1%, 37/56) or cutaneous lesions (25.0%, 14/56). We confirm that a single IFAT N. caninum IgG titre has limited diagnostic value regardless of the cut-off because healthy adult dogs exhibit a range of titres.