This was discussed at the inaugural meeting last year to set up the Rachuig commitee (or whatever it is called) and the general consensus was that Mackay was an NQ centre and Rocky an SQ centre. Considering that Rocky is 700 km from the Qld/NSW border, 730 km from Townsville and 1100 km from Cairns, that would seem to be the intelligent way to split the State. There is also a reasonably close spacing of centres from Rocky down, ie Gladstone/Bundy/Maryborough&Hervey bay etc, whereas the next centre from Rocky northwards is Mackay, 330 km away.
I am surprised there wasn't (or may not be) an 'official' dividing line set where the NQ zone was created. Perhaps NQ can set its own eligibity requirements in terms of residency.