There is no easy way to compare car insurance companies, as the cost will vary depending on a number of factors, and who is cheapest for one person may not be the cheapest for another.
In order to be sure of getting the best deal you will need to get a quote from each provider in turn. It is worth noting that level of cover, add on's such as breakdown cover and discounts for members or multi policy holders can also affect which provider is offering the overall best value.

Car Insurance

Provider	Policy	Insured Value	Liability Insurance	Glass Cover	Rental Car	Additional Info
AA Insurance	RoadCover Plus Self	Agreed Value	$20 million	Yes	Yes	Additional $10 discount for members and $30 off for online bookings
AMI	Private Vehicle Cover	Agreed Value	$20 million	Yes	Optional
Ansvar	Motor Vehicle	Agreed Value	$5 million	Optional	Optional
BNZ	Supreme Motor Vehicle	Market Value or Agreed Value option	$20 million	Yes	No	Optional Breakdown cover
Fin Tel	Full Cover Car Insurance	Market Value	$25 million	Yes	No
Kiwibank	Extra Cover	Market Value	$0.5 million	Yes	Yes	Optional Breakdown cover
NZI	Echelon Motor Vehicle	Market Value	$20 million	Yes	Yes
Tower	Tailored Car Insurance	Agreed Value	$25 million	Yes	Yes	Optional Breakdown cover
Vero	Motorplan	Agreed Value	$1 million	Optional	Optional
Westpac	Vehicle Insurance	Agreed Value	$20 million	Yes	Yes	10% discount is you have Westpac Home Insurance.

Car Insurance Example: Savings of $197 a year

In my own case looking for Comprehensive Insurance cover, the first quote came in at $504, the highest at $540 and the lowest at $343.

The difference between the cheapest and most expensive option is $197 for one car.
The difference from taking the first quote and the cheapest option was $161.