If you really want to threaten then duopoly, get in touch with Aldi and try encourage them to set up shop in NZ. They've done wonders for keeping Woolworths and Coles honest.
The Aldi shops I've visited in Aus have been okay-ish, but a lot of cheap "off-brands" and not what I'd consider a good selection of quality products., the store shelves also padded out with cheap and nasty appliances etc.
I knew a fellow who co-invested with Gubay in the attempt to set up 3 Guys in NZ in the '70s. That was a horrific decision - last I heard he skipped the country leaving a trail of debts - and truly disappeared - AFAIK never to be seen again even by his immediate family. I believe Progressive (Countdown) bought out the assets in a fire sale.
Aldi did plan to open in NZ, they had a .co.nz website with an "opening soon" message in the mid-late 00s, that then diverted as it does now to the .co.au site.
Would Aldi work in NZ? I doubt it - and I think Aldi doubt it too.