For anyone in Sydney, Genuine Edge in Lidcombe sell 500mL and 5L bottles of ACDelco Optikleen, and it's 20% off for a few days: https://www.genuineedge.com.au/acdelco-optikleen-windscreen-washer-concentrate-5l
They don't ship the 5L bottle, so it looks like I might be buying the little one from them. Can't spot anyone selling it off-the-shelf in Canberra.