Err well I don't really have any statistics either, so really I can't give proof or anything.
The feeling I get is that most mainland chinese people are rather ignorant of simple manners (this ranges the whole spectrum), at least compared to local NZers or NZ Chinese/Asian families.
There is admittedly a noticeable proportion of Chinese criminals, espicially in relation to drugs and money laundering (one of my mainland chinese friends has an older cousin living with them, and the police have been over to their house a couple of times snooping for drugs apparently) -- but these incidents (murder, kidnapping etc) imho is quite sparse. It's a tad harsh even with statistics to say that mainland chinese people's biggest contribution (excluding the student fees and such) is criminal activity though.
I can't speak for mainland chinese behaviour in public, as I spend most of my time still in high school -- BUT even some mainland chinese students are sort of rude or childish (at my school however koreans are perceived as being a tad more dodgy, espicially as there's so many). Taiwan, singapore, HK people are usually seen as more... friendly (right word?) than mainland chinese. I'd think that's because they're more "western" and would integrate with NZers more easily, but there are countless societal, educational and cultural aspects on why ML chinese are less polite/more rude.
It's just one of those things.
(Bladerunner, when you said "going crazy with cars," you did mean driving crazily in cars right?)
Also, just for fun; a few weeks back there was this chinese (or at least an asian that could speak mandarin) dude standing in Queens St in auckland CBD -- basically the times square of NZ -- with a big sign saying something about the lines of "90% of Chinese people is rude/lacking manners, 90% of chinese women is<insert rude word here>, 90% of chinese people is pigs". (incorrect grammar his writing, not mine)
He was asking for donations to keep his protest up, and quite a few people actually tipped coins in. The police were called quite a number of times, but for some reason they never arrived.
Finally another chinese person (3 days later) ran up to the protester and took the sign off him, running off with it. A security guard stopped the "theif" and the police finally arrived.
Afterwards there were calls for an internet manhunt of the protester. Not sure how far that got though.
Just an interesting tidbit, I feel it's kind of relevant to this discussion on manners and behaviour from mainland chinese.