There are so many variables, it's best just to call centres that you are interested in and ask. For example, some centres include nappies and food, other centres include one or the other, and then others don't include either.
Most daycares have part-days which are usually 9-3 or 9-3.30, anything outside of those times and you'll be into paying for a full day. Most daycares accept bookings for anything upwards of 2 days per week, but if you need 4 days then actually it's probably cheaper to just book for all 5 days.
There are WINZ subsidies available to help with costs but your entitlement will depend on your household income and how many hours you work per week, so don't forget to look into that.
As PP mentioned, there might be better options available to you if you're looking for flexible hours. Barnardo's is another home-based option. You can look around for other home-based care, or see if you can find a family to nanny-share with. Also, some councils provide creches which are way cheaper than daycare, but this will just depend on what you have in your area. We have one near us which apparently has a ridiculously long waiting list because it's so cheap.