4 month winter/spring trip

Normally I don't recommend buying a car for less than 6 months, but since you are coming in winter and so will be selling in the spring, I think that it could work. Have a look at www.trademe.co.nz This is NZ's go-to site for everything from used cars to share houses, jobs & lost cats. Once in the country you could also try Turners Auctions. Do read up on NZ law regarding Warrant-of-fitness (WOF) and registration.
Where do you come from? Have you checked what visa you require if not from OZ? If you are planning to work, you may need a work visa, and Immigration NZ will check your credentials.
Which airport are you arriving at? If you want to ski, probably best to start in the SI, but weather might be milder if you start in the NI for 6-8 weeks & hit the SI as spring blossoms. There is still skiing in September, although it might be starting to get slushy.

I think four months is long enough to warrant buying a (cheap) car. Presumably if you're working at least some of the time, then it will more than pay for itself, and you won't be "wasting" rental car costs if you stay in the one place for a week or more.

If you are plan to thoroughly explore the whole country, it might be best to start with the North Island in July, and get to the South Island by September, when it will start to slowly thaw out. I'm not sure when the ski season ends on the South Island, but there is extensive skiing available around Mt Ruapehu in the centre of the North Island.

You could consider heading north on arrival (presumably into Auckland), after buying a car there. The Bay of Islands are beautiful, and a week or more there will be a good introduction, even in winter. You might need to research the regulations attached to foreign nationals buying, registering, and insuring a vehicle in New Zealand.

In terms of reliable Internet, you could invest in a mobile broadband dongle for use when you cannot access free WiFi. You can easily buy a cheap car online (subject to whatever the rules are), but if you are fairly inexperienced in this area, it might be better going to a second-hand car dealer - but never pay the asking price - always aim to haggle the price down by at least 40% if you can.

In terms of itinerary, you have more than sufficient time to see essentially "all" the country ... but where you concentrate your time will depend mostly on your interests and priorities. The South Island is pretty straightforward ... you can circumnavigate the island either way, and see just about all of it.

The North Island is excellent too, and I would not neglect the two east-west "extremities" (meaning Taranaki/New Plymouth to the west, and Gisborne to the east).

You can even drive right round the isolated East Cape region as well if you are interested. But the centre strip is compelling and interesting - from Coromandel Peninsula / Tauranga through Rotorua > Taupo > Ruapehu > Wellington (via Napier/Hastings).

Hi Kereru

I do have a working holiday visa which is eligible till November 2016 since I have not used it yet. I'm
coming in from Singapore and will be working remotely for the company I'm currently working at....:)

Hi ianw6705

I am planning to fly into Auckland to start off. I think I will spend about a week or two there till I settle my car n explore it up before heading anywhere just to my feet on the ground for the long trip ahead. Thank you for the suggestions on the places to visit and the second hand dealer, I shall look out for those and try to educate myself as best as I can on the regulations, and cars in general..:)

Also, may I know what you mean by extremities when it comes to Grisborne and New Plymouth?

I just meant the most westerly point (New Plymouth) and the most easterly point (Gisborne) of the North Island, that most travelers either miss on a short trip, or remain unaware of their attractions. The East Cape drive (from Gisborne around to Whakatane) was nice, without being exceptional ... but given the time you have, why not? You would need to check the availability of Internet services right round though, if this is important to you on daily basis.

Ahh kkie......i misunderstood the extremities portion...facepalm..:).

Don't rule out small hostels either. There are some wonderful ones in nz. Check out the bbh site.

Indeed ... although we preferred YHA Hostels because of their consistent quality, and while we stayed in a couple of very good BBH ones, their standards are rather more patchy. But you can always check reviews.

@neverwinter @ianw6705

Yeap, noted..:D