I've repotted several of my natives because of the mild Sydney weather and all of them are looking okay. Callistimons are usually as tough as old boots and as long as the after care is good, if the tree were mine I would probably do it now. I have one callistimon, a collected tree, that is going to get the treatment Monday because the pot is nothing but roots. That is, IF the forecast is for continuing mild days... I'd do it. Thousands might not agree but as long as the tree agrees. Also, I am not worrying about putting a newly repotted tree in the shade. Full root prune and directly on the sunniest bench to get the most daytime warmth to encourage root growth.
Sounds like your Winter this year is much like ours: mild enough that the plants are keen to be budding a month or two early. I can only echo MelaQuin's advice about your Callistemon.