The pertussis epidemic experienced in NSW in 2008–2009 was likely to be in part due to changes in diagnostic practice since 2007, which amplified disease notifications. We used population-based seroepidemiology as a less biased means of interpreting age-specific pertussis infection patterns in NSW from three serosurveys undertaken in 1997–98 (during an epidemic), 2002 (post-epidemic) and 2007 (inter-epidemic), using a standardised pertussis toxin IgG enzyme-linked immunosorbent assay (ELISA). There was a decrease in the proportion of high anti-pertussis toxin IgG titres (>62.5ELISAUnits/mL) across all age groups in the 2007 serosurvey compared to the previous two serosurveys. In the 2007 serosurvey, the proportion of undetectable (<5ELISAUnits/mL) anti-pertussis toxin IgG titres increased in many age groups. The seroepidemiological profiles of the three serosurveys demonstrate fluctuating immunity profiles related to changes in vaccination schedules.