Auckland is New Zealand's biggest city. It is illegal to even walk on any of the motorways, including the on-ramps ($200 instant fine).
From the airport
It's easy to hitch a ride to the city center (instead of paying 15 NZD$ for the bus). Just walk out of the airport, go right and find the Shell station and ask around.
South towards Hamilton, Taupo, Tauranga, Wellington
In Auckland, you can either try to get a ride from in the city or try to get further south on the train to Papakura from Britomart ($8 without a HOP card). It'll probably take a couple of rides for you to get out of the city. If you get someone who can drop you in Bombay Hills, you'll be sweet! Buses are now pretty expensive to get to Bombay, so it may not be worth it unless you get lucky with your tickets.
There's a service station, but it is not as easy to get to as once before (the hole in the fence is gone). Your best bet is to go start hitchhiking and walking down great south road, and someone will likely stop for you within 20 minutes.
Train to Papakura and then caught the bus to Drury which is a few minutes down the road. There's plenty of space for cars to pull over before the go onto the motorway from here. There isn't a heap of traffic going onto the motorway but there's so much space. Waited ~5 mins and got a ride to Rotorua.
I actually now think this is the best option, as it saves you a few bucks and you have to sit on a train for an hour otherwise anyway. You'll likely be dropped in Papakura.
- Walk to the end of Hobson Street, where it crosses Pitt Street. Cross the road to where there is a left only turning lane onto the motorway. Stand on the pavement about 3-4 meters away from the lights. The cars almost always have to stop here before going onto the motorway. I've used this spot twice and it only took 20 minutes each time.
EDIT on 21/06/2021: the author must have meant that you should stand at the Pitt street, right at the left lane that is turning to the motorway.
At the on-ramp at the top of Symonds Street, close to the CBD. Walk down the on ramp and there is a big footpath you can stand on and plenty of space for people to stop. EDIT on 21/06/2021 - the Symonds street option seems useless now as the pavement is fenced now and as soon as the fence ends, the motorway begin. Hence there isn't any spot there for the car to pull over.
Apparently there's a gas station on the highway as well. There is also a Countdown petrol station right outside of the airport (closer than the Shell station) where a lot of the traffic seemed to be heading north.
There is a service centre near silverdale turn off on northern motorway before orewa and the toll road. If you can hitch from here there is a good amount of traffic going up far north.
Another good option is to take a bus to Orewa (which is worth visiting on its own) and hitch from there, Buses leave from Wellesley st West, near the sky tower. Take the 994 or 895, a few buses will take you there just read the timetables. Also you can leave from the north shore no problem, bus' leave from takapuna or Akoronga station, which is a bus station right on the northern motorway. Itll cost you maybe 4 dollars to get to Orewa (one can say that theyre going to an earlier stop - Albany for example and itll cost less and god knows nobody will check) Once you get to orewa there is a really good road to hitch from -Hibiscus coast "highway" (its really just a road, there's lots of space to pull over. This road is long and straight and near the beach, To go north head towards the hill, there is a dairy with a car parl where I often hang out and wait for lifts. Once you get going here it's pretty easy to go anywhere north, New Zealand is one of the easiest places in the world to hitch hike, you dont even need a sign, just chuck your thumb out and look friendly.
There are some BAD SPOTS to get rides from in Auckland. So you may wish to avoid the following;
Pukekohe - though it is close to the motorway, it's hard for people to stop
Hobson Street - although it looks like a good spot, it's better to walk to the intersection with Pitt Street as above. I think there's too much traffic for people to safely stop. I actually spoke to some guys who waited 6 hours here without getting a lift.