Hi Searcher,
July is within the winter period for NZ. To be frank, during July to Sep, the road is a bit slipperly due to "Black ice" for certain places. They called it "Black ice" because the ice on the road which is quite slipperly and you won't be able to notice it easily. Therefore, you will need to drive extra carefully, especially, cornering and overtaking. Also, you have to look at the road sign because some route might be closed. Definitely, places with thick snow, you must chain the tyres.
However, during this period, most of the motels and other attraction entrance fees are very cheap. You can get a night stay at only SGD55. It's considered off-peak.
For me, I took 15 days to complete the journey from North to South. But only explored 50% of its attraction areas. NZ is very big and it's as big as Malaysia. Everyday, I spent almost 6 hours on driving on average. Sometimes it could be very tiring. But the weather and the scenery would keep you awake all the times.
Driving in NZ it's as easy as ABC. The sigh boards are very comprehensive, of coz a lot better than Singapores'. Despite that, you would still need someone to read map for you, especially, when you reach cities like Auckland, Wellington and Christchurch. You may get lost somehow.
If you arrive at Auckland, I recommend you drive up to Paihia. There are a lot of beautiful beaches and activities. please refer to previous thread that I mentioned before.
South of Auckland, Rotorua and taupo, which has a lot of hot mud pools, Moari village tour, Natural spa, and lakes. If you have time, you can drive to Napier, also has beautiful beaches and enjoy the first sunrise.
Afterthat, you can head down to Wellington. Don't forget to take a walk at lambton street where it has a lot of shops along the road.