You are so lucky...
I was able to get 930mbps down back 2 or 3 years ago when gigatown first introduced, and the speed drops as more and more people installed fibre on my street(or in Dunedin?) Now I will be pretty happy to see above 300mbps down :-(
This is _NOT_ the reason. Chorus make the data publicly available about PON port utilisation and it's not a problem anywhere in the country. It _MIGHT_ be an issue with your ISP not maintaining handovers but honestly for a top tier ISP, that's very unlikely as well.
Cheers - N