> how do you know what MHW is at any particular place
Check the relevant chart. I've just taken a look at two AUS charts
AUs 621 says: "overhead clearance heights are above Highest Astronomical Tide, all other height are above Mean High Water
Aus 379 (which does not have any clearance heights marked) just says the latter.
Both maps include a panel which shows " Heights Above Datum" for MHHW, MLHW,MSL, MHLW, MLLW at the local Tide Stations.