After I noticed on Voyager's website the free speed upgrade appears to finish at the end of the year, I contacted them.

Their response FYI

"Thank you for contacting us about this.

The 200/20 upgrade offer applies to all customers who are on an Enable circuit (within Canterbury). Enable Networks had a plan for the Christchurch Fibre development and they exceeded their goals well in advance of their planned dates/times. Because of this, they offered all Canterbury providers discounted rates on their 200/20 plans. This offer finishes on December 31st, 2019.

We will likely have a newsletter sent to our Christchurch customers on the 200/20 plan indicating that the price may increase at the end of the promotional period. Closer to the end of the promotion, you will likely get the chance to choose which plan/price you would like to stay on ($79 for 100/20 or $99 for 200/20 - the standard price for 200/20 connections). However, this has not been confirmed yet. Ideally, we'd recommend keeping an eye out for correspondences from our media team 30, 60 or 90 days prior to the end of the promotion for more details."

I've been very happy with Voyager so far(switched to them as the free speed upgrade kicked in) but for $99 I can get Gigabit elsewhere...