Based on a previous offer made by slingshot & listed here: https://www.cheapies.nz/node/39675, I signed up and now received and email notifying of a price change (for me increasing by $9.05/month).
- You can call them for them to honour it and give a credit countering the same amount for the remaining period of your contract.
- You can call and change your provider without incurring early exit or notice period fees (you have to give 30 days notice)
- You can surrender and pay the price increase
Some of these answers are online:
https://help.slingshot.co.nz/hc/en-us/articles/1965263537845…
Now I'll be checking if there's a better deal or accept the credit.
Update: Looks like I'll be joining 2degrees, they have 4 months free and $10 if I have mobile plan with them.

A list of codes still usable at Slingshot. 300/100 is the speed chosen in terms of cost.
https://www.slingshot.co.nz/plans/builder?byod=Yes&frequency=Monthly&voucherCode=CODEHERE
or
https://www.slingshot.co.nz/plans/builder?vouchercode=CODEHERE
BBCOMPARE22 (particular link)
$22 off + 10 credit for 12 months = $65/month
BBCOMPARE20
$20 off for 12 months = $77/month
4MNTHSFREE
4 months free then $97/month
StuffFibre3Mnth
3 months free then $97/month
SSGLMP3M
3 months free then $97/month