You may want to look at
www.woolworths.co.nz (you may have to register first)which allows you to see what grocery prices are like.
We actually find them more expensive than the UK on most things especially fresh fruit and veg. Although it is cheaper than Oz!
We've found that most weeks on bills, including fully furnished rental in Wellington ($480) that we are spending around $550-600 a week then around $80-100 on food.
Eating out is fairly inexpensive as long as you dont go for posh nosh and you'll probably look at about $60-80 for two to eat out in a decent place at the moment.
You mentioned water costs and if you rent in NZ the Landlord covers this not the tenants which is pretty cool.
Another expenditure to look out for is if you do rent through an agency some of them charge you a fee. In the UK this fee is paid for by the Landlord but here it's us poor tenants who get stung. This is usually one weeks rent plus GST!!!