Fig.1 - The benchmark at mean tide
" Recent publicity arising from a study by the Universities of Southampton and Tasmania [1] of an old sea level benchmark at Port Arthur, Tasmania, claims `dramatic' sea level rise of about 13cm since the mark was first struck back in 1841.
These claims are based primarily on statistical probability models and are unsupportable when viewed in the context of other physical and documentary evidence available surrounding the benchmark. The study claims the benchmark was originally set 44.5cm above the mean level of the sea (as it then existed in 1841). Since it now sits at 31.5 cm above (see fig.1), the difference - 13cm - is claimed to be evidence of sea level rise.
However, the man responsible for putting the mark there, explorer Sir James Clark Ross stated explicitly and several times in his 1846 book [3] that the mark was placed at MSL (as he estimated it to be), not at a point 44.5cm above, near the high tide point, as claimed by the study. Other evidence surrounding the original placing of the benchmark is less clear, but we do have one positive measurement of where the benchmark stood relative to sea level taken in 1888 by the then Government meteorologist, Commander J. Shortt R.N. He found the mark to be 34cm above sea level - only 2½ cm different to its current position [7] [5]."