Understanding of the datums

AHD (Australian Height Datum) is also functionally the same as MSL - mean sea level. This is a height datum point = 0m

However, tides are based on LAT - Lowest Astronomical Tide, which is effectively the lowest tide of the year and this datum is also 0m. (But a different 0m).

We should never see a low tide with a minus sign. We do, but this is due to meteorological factors like high pressure systems, off shore winds etc.

Based on the NSW Ocean and River Entrance Tidal levels 2104-2015, the difference between AHD and LAT for the Tweed River is 0.893m

So if a tide chart shows a high tide at the Tweed River mouth of 2.0m, the water level would be 2.0 - 0.893 = 1.107 AHD.

I hope this helps.