Quick update from me here.
I believe we have found the issue for both Napier and Kapiti for Bigpipe customers. Thanks for bringing this to my attention.
It would appear that (unknown to us) Chorus seem to take your traffic on a grand tour before it is handed over to our network - and is working as designed from their perspective. Obviously not ideal and we are exploring options to see if we can avoid this. Can't make any guarantees at this point but we are working on it!
This has made me smile (sort of).
Would be interesting to know if all other RSPs are impacted or if this is special to Spark.