Most youngsters I know do the Southern Hemisphere in twos or threes. The last was my friends daughter with friends. They did SouthAfrica in Capetown and Safari reserve with cheetahs. .
When going to Johannesburg being streetwise straight into the airport and off to next destination. Next Australia then New Zealand return via Thailand.
Like has been said if you were visiting London you would be more at risk than New Zealand.
If with friends agree a safety plan such as don't split always stay together. Even if a lad who likes you looks like your favourite pop star. Now thats a challenge.
Go online check advice forums and websites.
I reckon after what has happened
there will be lots of help in NZ for single back packers and others.
Take note of the full story of the tragedy when the full facts emerge and learn.
Enjoy and just stay streetwise.