Will cost plenty more to replace all generation ...

Plenty more than what? If we had to replace a handful of large monolithic coal-fired plants with something similar you'd have a point, since the public would foot most of the bill. A renewable grid would be powered by a huge patchwork of public and private generators - the currently-installed, privately-owned array of grid-connected rooftop solar panels and inverters is a good example of how costs would be shared. Rooftop solar has already displaced the building of new coal baseload plants. A combination of individual consumers investing in their own electricity supply and small businesses taking advantage of local demand is all that's needed.

... new transmission lines and grid support in Australia with a renewable only system, even if its is in fact possible to have a grid reliant on 100% renewable generation.

Energy transmission systems exist in their current form to reliably deliver energy without interruption, 24/7, while withstanding daily demand cycles and occasional transient interruptions. When storage matures as a commercial technology, the demands on the grid will be greatly reduced - long-distance transmission will see a smoothing out of its load profile, since it will largely be trickling a constant stream of the cheapest power to local storage systems. Storage is a game changer for both the traditional baseload+peaking paradigm as well as any distributed renewable system. There's no reason why the current grid would need to be upgraded with anything more than commercially-provided and operated storage.