I found the website is a great value compared to other car hire websites (Priceline, Expedia, Discovery Car Hire etc.)
This offer includes:
Rental price discounts vary from 10%-20% off.
Offer valid on rentals throughout New Zealand.
Offer valid on almost all car categories.
Offer valid on multiple car rental companies. (Check out those with discount tags)
One-way fee included!
Unlimited rental days and mileage!
Offer valid from Mar 1, 2018 - Jun 30, 2018 GMT.
The discounts are automatically applied.
Other promo coupons can still be applied after this discount;
Seriously should give Go Rentals a go. We went through them on our 3 week adventure through the South Island. Friendly service, clean & new cars (ours had 5000KM on it) and offer a referral card that gets you alright discounts on touristy things.
Quick search over 24-30th reveals Go Rentals are cheaper overall compared to the website you supplied. The real bargain here might be the inclusion of one way car rentals.
(Also - get travel insurance that includes the car rental; ours got damaged during a break in at Te Anau and these companies charge stupid amounts)