I am having the same issues. Was watching on demand just yesterday, but tonight, its being a right pain.
just got this... EMAIL..
Thanks for getting in touch!
FreeviewPlus have recently released a large update that combined all 3 OnDemand services into one app, this app requires you to create a Freeview account (this is separate to your TVNZ OnDemand account).
You can register for your Freeview account with the same email address you use for your TVNZ login; however your TVNZ account is not linked to your Freeview one (we don't have the right to share your user info with one another).
If you continue to have issues, don't hesitate to get back in touch with us or with Freeview!
You can get in touch with Freeview here: http://www.freeviewnz.tv/support/contact-us/
Sorry for the inconvenience!