AHD (Australian Height Datum) levels are the height above mean sea level.
If you know what the AHD level of one of your Revit levels is meant to be you can set it to be that level.
All Revit levels above and below this level will also be adjusted.
PROCEDURE
Go to a floor plan view whose associated level is the level you want to change.
Make sure there is a model object whose top plane aligns with this level - a floor is best. (Annotation objects like lines won't work).
From Manage tab, Project Location panel, Coordinates, Specify Coordinates at a Point command.
Pick somewhere on the model object, change Elevation to the AHD value in millimetres you want.
Leave values East/West and North/South and True North angle as they are. (they may be blank if you didn't pick a point)
CHECK
Go to an elevation or section view, check the level height now matches the AHD you entered.
Note the level reference may show the height in metres, not millimetres
If it doesn't check the level tag is reading the correct elevation base:
Highlight the level reference
In properties window hit Edit Type.
Make sure Elevation Base parameter is set to Shared.