I asked them again:
I've just checked your T&C. A standard liability insurance with up to 5.000.000NZD coverage is always included in the standard rental rate which is calculated on your website, but with a excess of 2.500NZD, correct?
And they replied:
You are correct. you can also purchase our full cover which reduces liability to $0 for only $25-29/day depending on the vehicle type.
Sadly the vehicle I had in mind is sold out now. But maybe this information help someone else.