It's generally a VMC vs IMC thing. Yes, runway heading and runway track are different in Australia.
"Cancel SID maintain runway heading visual". Note the "visual" in there - it's now a visual departure. The pilot(s) are to maintain their own terrain separation up to the minimum vectoring altitude. "Visual" is also a required readback. It can be handy to know the MVA - or just ask. When you maintain runway heading you don't compensate for wind.
"Cancel SID maintain runway track (180 degrees)". In this case as Mr sabre pointed out, you fly the climb gradient on the SID you were initially issued while tracking straight ahead. The SID is providing your terrain separation. You do compensate for wind. Again as per the previous post, the SID must allow you to fly runway track up to the MVA - so not every SID may be cancelled in this manner eg some SIDs for smaller aircraft have an early turn to get them out of the way faster.
You may also be given "leaving 2000 (or whatever the MVA might be), cancel SID turn left heading 180". This one probably most often used for weather avoidance where a pilot can't/won't accept a visual departure eg "Cancel SID turn left heading 180 visual". Another instance where knowing the MVA is useful. A visual departure generally allows you to turn earlier than the MVA. ATC won't (well, shouldn't) issue a visual departure if you can't achieve it.