TVNZ does not currently support casting to Android TV based devices with Chromecast built-in such as the Freeview Streaming Device.
TVNZ constantly evaluates the devices that are supported, and we here at Freeview will work with TVNZ to ensure TVNZ OnDemand is available to viewers.
In the meantime, you can enjoy all the good stuff on TVNZ OnDemand, by downloading/installing the TVNZ OnDemand App directly onto your Freeview Streaming Device.