I'm guessing @izogi may have hit on the majority of the issue with:
" it might be because there are an increasing number of transient people in NZ with better internet access, who could all disappear if the tourism boom drops off."
Between smart phones and free library internet access the amount of time tourists spend on the internet must have increased
for 30 years i've always been tramping if i've been in the mountains, hiking was never a nz term, we all called ourselves trampers. i wouldnt say there is a dividing line in the type of tracks your on , or whether you're on or off track, if you're off track you're stil tramping but could be called a bush basher if there is bush..
I'm wary of trying to get too specific about detail since the whole thing is so ambiguous the more you look at it, which I guess is why I've mostly been commenting on trends of what's popular.
But if I was going to try and come up with a short and sharp description, I think it'd be that..
Tramping is what you think you're doing.
So is hiking, for that matter.
@FrankB has it right. If I'm going out on a day walk, or an overnight or more, I'm tramping. Its what that activity is called in NZ.
I often read blogs of overseas visitors. They often have a section pointing out the meaning of tramping in NZ - which in their view is hiking, and not getting drunk and sleeping around :)
Tramping is clearly an NZ term, but as Brit who's been here 8 years it's one I most comfortable with now, and use regularly.
As for hiking being an overseas term, it's probably more American than anything else, it's not used that regularly in the UK, hill walking would be the most common in the UK, and perhaps backpacking.