Done some digging on this and now I’m even more curious.
91.5 was originally to be allocated to the first new commercial radio service in 2000 - what became Nova 100 (3MEL) instead launched on 100.3 displacing 3YYR Geelong, now 94.7 The Pulse (3PLS). This limitation RE: ABNT3 Launceston is outlined in the October 2001 LAP variation. (link)
This service is required to protect the reception of ABNT3 Tasmania North East. The specification is limited to a maximum ERP of 22 kW on the bearing of 170° until the clearance of ABNT3 Tasmania North East or test transmissions conducted by the licensee confirm that operation in accordance with the above specification does not cause interference to the reception of ABNT3 in areas where an alternative television channel is not available.
By 2004, ABNT had vacated VHF channel 3, and this stipulation was removed from the three affected Melbourne frequencies - the then-unallocated 91.5, now Smoothfm (3PTV); 89.9 Triple Seven, now TheLight (3TSC), and 90.7 SYN (3SYN).
This is where things become confusing. The April 2004 LAP variation licences SYN at the conventional maximum ERP of 56kW in line with the rest of the Melbourne-wide FMs. (link)
Bearing or Sector (Clockwise direction) Maximum ERP 0°T - 30°T 56 kW 30°T - 150°T 2 kW 150°T - 240°T 56 kW 240°T - 250°T 16 kW 250°T - 360°T 56 kW
This is consistent through the subsequent variation in 2006 (link), and is still present in the most recent LAP from 2017 (link).
Yet, the licence image for 3SYN available on ACMA’s RRL database (link) instead outlines the station’s radiation pattern as the following:
Bearing or Sector (°T) Maximum ERP (W) 0 - 30 35 000.0 30 - 150 2 000.0 150 - 240 35 000.0 240 - 250 16 000.0 250 - 360 35 000.0
Note that 35 kW in the 150°T - 240°T bearing is greater than the 22 kW limitation required pre-2004 to prevent interference to ABNT3, which rules out my earlier theory.