I travel from Chch to Southland regularly
I fuel up at McKeown's in Hornby (Halswell Jctn Rd about 300 to 400 meters from SH 1)
Price $1.509 per litre. (1 Nov '18 for Diesel)
That's about 27 or 28 cents less per litre than the normal con artists.
No pump attendant,
No lolly shop,
Don't need to sign up for any minimum amount per month or need to be a Farmlands card holder etc,
Don't need to join any club.
If you have an account, then greater savings can be had if usage per month is a certain dollar or litre amount.
Just need a plastic money card of some sort.
Some places have 91 octane fuel but you might be disappointed in the 91 price. They seem to be geared up more for trucks and diesel users
They are in many but not all towns and cities in the South Island.
Some of their pumps are in Truckies depots but still operate by serve yourself system.