How safe is NZ? Pretty safe. The Japanese once came close, but never made it down this far. The French bombed a boat here once, but we live in a benign strategic environment, apparently.
We already have democracy, so the US is unlikely to invade at random.
Hope that answers your question.
The Japanese got a sub into the Wellington harbor during ww2. It was never shot at because the officers in charge of the guns at Wrights hill were on a tea break.
Hitchhiking: NZ is probably one of the better countries for hitching: relatively safe and I've never waited long for a ride. Carry a cellphone and make a show of texting the license plate to someone if you're worried.
Camping: There have been a small number of widely publicised murders of people staying in campervans in isolated spots. I wouldn't worry about it unless you're especially paranoid.
Driving: Remember kiwis drive on the left. The road signage is excellent and the road surface is usually good, but many roads, even main roads, are only one lane each way and often windy. So if someone is following you it's polite to slow down and let people overtake you if there's a straight stretch. Using cellphones while driving is illegal. Indicate for 3 seconds before changing lanes or turning, there are plenty of motorcyclists fanging about enjoying the scenery. If you're not used to driving on gravel roads or beaches, take advice and take it slow.
Also, there is an unusal left turn rule that is the exact opposite of other left-side countries like the UK & Ireland. When turning left into a junction, if there is a car going the same way (eg, turning to their right and facing towards you) - they have right of way. You have to give way. The NZTA have further information for overseas drivers. We recently joined the civalised world in respect to our turning give way rules. Here is a summary of the new rules