Small datapoint: On my last ten flights arriving in Australia (seven flights originating in the US, two NZ flights originating in New Zealand, and one NZ flight from the Cook Islands), I've gone through the spraying process once (arriving from SFO on UA).
Not a common occurrence. Since there are a lot of NZ flights to Australia every day, I assume that they make sure to fumigate the aircraft according to AQIS specifications to avoid the on-arrival spraying.