Hello and welcome.

I would agree that it's most likely been plated.
I own an Australian "silver 2 cent coin" that my Dad made years ago as a chemistry demonstration. It's been plated with zinc.
On plated coins, there is, almost always, a small gap or hole in the plating, where the wire was attached during the electroplating process. On your coin I can see this hole, it's at the top of the reverse (flower side), on the rim - there's a long strip where the bronze is showing through the plating.
The easiest test for a "wrong metal planchet" mint error is to weigh it; a true wrong metal planchet error will not weigh exactly the same as a perfectly normal coin of the same type. So get yourself a balance capable of measuring to two decimal places, place your coin on it and weigh it, then get a regular NZ 2 cent piece and weigh it too. If the weights are the same, then it's a plate job.
Plating coins like this is technically a form of counterfeiting and therefore illegal, since it is possible to try to pass the coin for one of higher denomination (in this case, a 2 cent piece is about the same size as an old 10 cent piece), which is probably how your dad acquired it in the first place.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis