In the past, Spark charged $9.99 per month for "Ultra VDSL" on top of the price for ADSL. However, their current plans have ADSL and VDSL at the same price [1].
If you have changed to a current Spark VDSL plan (and are not on a fixed-term contract), you should be paying the current price (i.e. no $9.99 surcharge). Unfortunately it seems Spark may only stop charging the extra $9.99 if you contact them and ask.
This was the case for "kiwipearls" [2] and "Jase2985" [3]. Both of these Geekzone members were paying the $9.99 surcharge - when they spoke to Spark about it, they were able to get the surcharge removed from their bills and obtain a refund of the surcharges paid since November 2016.
I too have contacted Spark to get the $9.99 surcharge removed from my bill. Unfortunately I was not able to get previously-paid surcharges refunded, but at least I will not be paying the surcharge from now on.
Make sure you are not paying an avoidable "Ultra VDSL" surcharge either.
[1] https://www.spark.co.nz/shop/internet/plans-and-pricing/
[2] https://www.geekzone.co.nz/forums.asp?forumid=39&topicid=205529&page_no=2#1700617
[3] https://www.geekzone.co.nz/forums.asp?forumid=39&topicid=205529&page_no=2#1700809