Heh, this sort of thing isn't something that a couple of politicians can agree amongst themselves. You will need parliament in both countries to pass laws to enable it. And if NZ is serious about its own obligations towards the rule of law, UN treaty obligations around treatment of prisoners etc, and to ensure that its own HC's sentence is properly upheld, it should make independent inquiries of how he's going to be held and secured etc.
Get excited if and when it happens. These things aren't just about soundbites and what makes people feel good.