Im also from Perth and I roasted yesterday afternoon when it was quite cold, about 12 degrees. I roasted some Ethiopian Harar and Hawaiian Kona. I found that the cool weather also extended my roast times, but in a beneficial sense. The Harar took just under 10 minutes to roast to the very first snaps of 2nd crack. This was using a B&D popper with chimney, roasting 120G of beans. I was agreeably surprised by the more appealing aromas coming from the double espresso that I usually have. It seemed less coarse than the previous batch which roasted to the same spot in just under 8 minutes when the weather was significantly warmer.
The Kona took about 12 minutes to get to the same stage and has also turned out nicely.
Interesting to see the very long roasting time for your decaf. My experience with decaf is that 1st and 2nd cracks have not been as noticeable as with full strength beans and I recall during some roasts hardly hearing any first crack, just a snap or two with 2nd crack being similar.
I guess the cooler weather would be a good thing for us Perth popper roasters (PPRs!) as longer roast times are supposed to mean a better resultant flavour profile according to the experts.
I havent done enough cold weather roasting to be really sure on this one