Your Whistlers are not native to NZ, they were introduced here from Australia as well as the Green & Golden Bell Frogs and the Southern Bell frog. It is against the law to keep NZ native frogs. Ive never seen a NZ native here any way so they must be in really remote places. I can tell males from females by the shape and colouring. The Female tends to be quite rounded (Fat..Lol) the Male is leaner and Ive found they tend to have more yellow colouring on the arms and legs and under body. The Females are mainly whitish underneath. Im no expert but Ive guest right with most of my frogs. Oh and male croaks
. That a sure sign.