A paper published in 1889 by Captain Shortt recorded the wording of the plaque, including the time the mark was struck and the height of the sea given by Lempriere's tide gauge. By taking a measurement of the height of the sea, and estimating what the tides were when the mark was made, Shortt determined that the mark was made near high water.
An article in The Australasian in 1892 also recorded the wording of the plaque. While almost the same as the version published in Shortt's paper, it differed in the time the mark was supposed to have been made, although both reports were consistent regarding the reading of Lempriere's tide gauge when the mark was struck. Taken on its own, the reported time of the striking would suggest that the mark was originally near mean sea-level.
Significant work has gone into determining which of the accounts is correct, including a current major study by a collaboration of international scientists, as knowing whether the mark was originally placed near mean sea-level or high water is crucial to being able to compare sea-levels of 1841 with today. This study has concluded that it is almost certain that the benchmark was originally placed near high water. The conclusion is based on other estimates of sea-level made later in the 19th century, and on the fact that, if the mark had originally been placed near mean sea-level, then the Penitentiary building would have suffered flooding every few years (there is no record of this having happened).
given the latitude, a 2 to 3 meter difference between mean high water and sea level is not surprising.
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