People may incorrectly perceive that their body weight or Physical Activity (PA) meets health recommendations; this provides an obstacle for change. In this study self-reported BMI and PA were assessed in relation to questions regarding perception of meeting weight and PA recommendations. Signal detection analysis was used to identify population subgroups. Study outcomes showed that 34.4 per cent of the 2535 adult participants underestimated weight health risks; low income, male participants who believed they were sufficiently active underestimated weight health risks the most (52.6%). A total of 23.7 per cent of participants overestimated PA health benefits; older age, low educated participants who did not believe their body weight was a health risk overestimated PA health benefits the most (58.3%). In conclusion, as weight and PA misperceptions are high, efforts are needed to reduce these misperceptions, so that behaviour change initiatives can be more effective.