Really... I am not surprised at all,

This is the only case they gone as far as a prosecution as the victims agreed to provide evidence,

There have been many cases in the past regarding debt bondage in the restaurant and escort industry that would fit the definition, its just often the victims were migrant workers and they were too scared to provide evidence....

Crimes Act 1961

http://www.legislation.govt.nz/act/public/1961/0043/latest/whole.html#DLM328579

98 Dealing in slaves

(1) Every one is liable to imprisonment for a term not exceeding 14 years who, within or outside New Zealand

(b)employs or uses any person as a slave, or permits any person to be so employed or used; or

Slave includes, without limitation, a person subject to debt-bondage or serfdom.

There have been any number of somewhat similar cases of migrant worker exploitation. New Zealand has it's fair share of low life's willing to exploit others for money.

I'm surprised it doesn't come up more often in all honesty, especially with the mess that has been made of residency visas.

https://www.newsroom.co.nz/news/a-useless-handbrake-is-released