Runway Track is used by night or non visual conditions below MVA
Runway heading is used for VFR or IFR on a visual departure.
All the references are spread around in AIP.. I dont have a current copy at hand but my saved electronic copy that was valid at the beginning of this year has this:
Phraseology GEN 3.4 - 51:
when a VFR aircraft, or an IFR aircraft cleared for a visual departure is issued radar heading instructions
Sl. (instructions) MAINTAIN RUNWAY HEADING [TURN LEFT (or RIGHT) HEADING (degrees)] VISUAL, CLEARED FOR TAKE--OFF
ENR 1.1 -11
para 4-8-1 Departure instructions may contain the following as required:
b. heading instructions;**
** Note: A pilot assigned a heading (including runway heading)
must not compensate for wind effect.
ENR 1.5 -36
10.4.4 In a radar environment,when aProcedural SID is cancelled before
take-off, ATC may:
a. require the pilot in command to depart in accordance with a
radar SID; or
b. issue alternative instructions that require the aircraft to depart on runway track using the climb gradient specified in the cancelled SID. In this case, ATC will use the phrase“CANCELSID, MAINTAIN RUNWAY TRACK (three digits) DEGREES”. Note: For the application of this procedure, the runway and radar SID tracks must be coincident up to the MVA.
Now for a bit of interpretation, if you are departing in non visual conditions or by night, below MVA, ATC can only vector you if:
1. You are flying a radar SID or
2. You fly the the first part of the SID eg "maintain Runway track" (which is what you do when you fly a SID) until MVA is reached after which any heading can be applied.
hope that helps
Wal in not so different Perth