Speaking to a staff member at our local JB Hi-fi, apparently confusion over the game's street date was the problem.
"We weren't really given a date," claimed the staff member, "so we just started selling them when they came in."
There are still issues with instrument stock, our local store has the Keyboard peripheral, but no Pro guitars.
"We don't have any Pro guitars," he said. "Some stores have them, but if you want the stringed guitars you should probably look into importing them - that's what I'm going to do."
It's interesting to note, however, that JB Hi-fi (our local store at least) is continuing to stick to the street date regardless, despite having the game in stock.
We asked EA for comment - they were aware of situation, and are considering it "a soft launch."