This was an observational study of 828 lactations in 542 mixed-age dairy cows that calved seasonally in a single, pasture-fed herd in New Zealand in 2008 and 2009. The study objectives were to: (i) document daily liveweight change (∆LW) before and after observed oestrus for cows subsequently diagnosed pregnant or non-pregnant and (ii) quantify the sensitivity and specificity of ∆LW as a test for oestrus. The sensitivity and specificity of ∆LW when combined with other commonly used oestrous detection methods was also evaluated. In cows that conceived as a result of service at detected oestrus, liveweight loss began 1 day before the day of detection and was greatest on the day of detection (−9.6 kg, 95% CI −11.3 kg to −7.8 kg; p < 0.01) compared with LW recorded 2 days before the day of detection. In cows that did not conceive to a service at a detected oestrus, the lowest liveweights were recorded 1 day before the day oestrus was detected (−4.3 kg, 95% CI −7.7 to −0.8 kg; p = 0.02) compared with LW recorded 4 days before the day of detection. The sensitivity and specificity of ∆LW as a means of oestrous detection were 0.42 (95% CI 0.40–0.45) and 0.96 (95% CI 0.95–0.97), respectively. When ∆LW was combined with tail paint and visual observation, the oestrous detection sensitivity and specificity were 0.86 and 0.94, respectively. Monitoring LW change holds promise to enhance the sensitivity and specificity of oestrous detection in combination with other oestrous detection methods.