Why are we still getting gouged at the pump?

I for some time now, like most of us, have noticed the incredible variance in fuel prices across Christchurch, so yesterday morning (Tuesday 30 July 2019), I decided to make a note of these difference.
Now to be fare I have picked on ‘Z’ stations for this comparison, but all the main players are just as guilty. I am also aware that there will be comments that some of these stations offer say 6 cents off per litre but is this really a discount or a ploy to suck us in? surly when offering a ‘discount’ you should not be putting your prices up to cover it!
- The Plams Diesel $1.489 91 $2.209
- Main North Rd Diesel $1.499 91 $2.219
- Harewood Rd Diesel $1.559 91 $2.219
- Blenheim Rd Diesel $1.559 91 $2.259
- Moorhouse Ave Diesel $1.559 91 $2.259
- Brougham St Diesel $1.559 91 $2.219
- Ferry Rd Diesel $1.559 91 $2.259
- Linwood Ave Diesel $1.469 91 $2.209
Yes, I know some of these stations are independent or privately owned but still there is a 9 cents per litre variance in Diesel and 5 cents per litre variance in 91
I can’t understand why there hasn’t been a full investigation in to these oil companies, I know the government get a fair whack in tax payments but for a country of hard working people, would someone please stand up and be counted and hold these oil companies to task.
As a foot note I filled up my diesel ute before I started to make these comparisons for just $1.399 per litre, so yes it does pay to shop around.