Just to throw in more anecdotal 2c, I’m on Vodafone gigabit fibre and this happens all the time for battle.net on release date for patches especially modern warfare. It’s highly likely whatever source activision-blizzard has for NZ throttles due to load and has nothing to do with ISP.
Come back a few days, even a day later and I’ll get full line speed (100-120MB/s) and be done in no time. If I try during peak time on patch day I’ll be breadcrumbing a 16~40GB download at 600KB/s~2MB/s.
Same deal with Win10 MS store games and the occasional uncommon steam game. Steam is great for testing as you can just go setting-> change to Aus NSW and see download instantly ramp up. Battle.net and MS store you need to VPN to sydney.
Big 80-120GB games like Gears of War 4/5 or Modern Warfare are no fun to download at sub-adsl speeds, just utilise the Aussie nodes (since they don’t/can’t seem to fully use them).