Originally Posted by sunnywa
The 1500ft thing for IFR is, in Australia anyway, based on the fact the lowest LSALT is 1500ft, even over the sea, and IFR aircraft cannot transit below LSALT.
I'm sure that you know that the Australian LSALT calculation is based on 1000ft above the highest point plus 360 feet for undeclared obstructions
AIP GEN 4.5 deals with NVFR, and
2.2.1 is for IFR/En route calculations. 1500ft is the lowest allowed LSALT based on rounding up from these figures.