A sediment accumulation record based on the levels of “unsupported”210Pb was obtained from a core taken from the centre of Wellington Harbour, New Zealand. Sediment accumulation rates (SARs) varied from 3 to 5 mm yr‐1. Before 1920, SARs were sufficiently high to dilute unsupported levels of210Pb to background, but had fallen below the mean by 1925 maintaining constant SARs until 1953. These data provide a higher resolution chronology of early 20th century SARs than was previously available. Comparison with137Cs data confirms that there was a significant increase in SARs between 1953 and 1967, but that these have remained fairly constant since 1974. Data from the two radionuclides are consistent after 1974, and210Pb data are consistent with other chronological measurements before 1953. Therefore, it is suggested that the two radionuclides are immobile in the marine sediments of Wellington Harbour and that differences between the two SARs are real.