I tried re-running the setup and this time around I paid a bit more attention.  It says "Connect Deco to your modem via an Ethernet cable" and then underneath the option "I don't have a modem."  I clicked that and it said "Connect Deco to a LAN port.  Connect your Deco to a network-providing devices LAN port, such as a router, switch or internet outlet."  I thought - aha - this is more like it, as I've connected it to the ONT.  I followed through the rest of the steps and chose Dynamic as the connection type, but it still said it couldn't connect to the internet.  I tried PPPoE but it had the same trouble.  I went into the advanced settings and there is a toggle to turn on IPTV/VLAN, which I did and then chose the NZ profile (SparkNZ/Chorus Fibre) The other options are Static IP, L2TP, PPTP, DS-Lite, V6 Plus, and MAP-E(OCN).

I then tried a few more times, and I'm not sure what I did, but it finally connected.  Success! But that's not the end of the story.  It was super wobbly, and kept disconnecting (keep in mind this is one unit connected directly to the ONT, and me connecting via wifi in the same room.  The Deco app was appalling as well.  But I was able to connect a second satellite successfully.  Or so I thought.  I moved it down to the lounge and it looked like it was connecting back to the main satellite okay, but then it disconnected.  So I brought it back down to sit right next to the main satellite - still wouldn't connect.  So I tried a reboot of the satellite.  Nothing.  A reboot of the whole network.  Nothing.  I tried a reset to factory settings for the second satellite.  It wouldn't re-initialise and the Deco app couldn't find it.  But when I went back to the main map it was showing there.  But the the main unit wouldn't connect to the internet. Sigh...

Suffice to say this has not been a positive experience with TP-Link Deco.  I'm seriously considering taking it back to Harvey Norman for an exchange of some other mesh system, though I've no idea what I would go for.  But for something which promises simple setup and configurability this is not what I was expecting.  I'm no moron when it comes to technology, but this just seems ridiculous.