I live in NZ too and have been dealing with electric bikes, so here's my 2c worth: (Adding to Robert's reply)

In terms of stability, adding a mid drive motor and bottle holder mounted battery puts the extra weight at the center of the bike. Whereas adding a hub motor tends to disperse the weight towards the front or rear, making it slightly harder to handle when turning. I did notice some difference when I tried them out. Not sure how it would handle in wet ground though.

In terms of ability to fit, most bikes can fit a hub motor because sizes are standardised around the fork and rear hub. However, you'll need to be careful of how much weight and force you're adding. The front fork is structurally weaker than the rear, and adding extra weight becomes extra force on the fork when going over bumps on the road. Depending on what the road quality is like, it may be a good idea to add a torque arm as support to avoid damaging the frame.

On the other hand, mid drive motor takes advantage of the bike's gearing (better for up hills with 300W), but this could produce more wear and tear on the drive train. Installation is more difficult as well, as most kits require special tools to remove the bottom derailleur to mount the motor kit. In NZ (where I live too), you can get that done for around $70 ~ $100 from BikeBarn / T7.

In terms of torque, the BAFANG BBS01 (mid drive) states >= 80Nm max torque, whereas the BionX system (hub motor) states maximum 50Nm. Not sure if this is general difference but don't forget the mid drive still has the gearing to amplify the torque.

I'm going to be fitting a BAFANG BBS01 onto my Merida Ride 100. If you're interested to take a look, feel free to PM me.

Hope this helps! And good on ya for cycling in NZ.