The Brisbane Line dates back to World War II. It was the alleged strategic line of defence drawn at Brisbane because it was thought that, in the event of a Japanese invasion, it would be impossible to contain the enemy in the north of the country and more effective to defend the more settled parts of Australia from Brisbane down.
The Queensland state election of 2017 produced the Gympie Line. Apparently the LNP warned Malcolm Turnbull that he would not go down well with voters above Gympie and that it would be better if he did not cross the Gympie Line.
This idea has been resurrected in the 2019 election but this time, with his blokey image and travelling in his bus, ScoMo has penetrated far beyond the Gympie Line. Whether it will do him any good or not, remains to be seen.