Your options are limited in NZ, You can get Diamond D130 discone from D130 Discone Antenna – Com-Centre Communiction Equipment Limited
call them before you go, (Richard is a grumpy bugger) or Icom https://rwb.co.nz/aviation/aviation-accessories/antenna-options/ah-8000. Both will fit your requirements. Also you need good quality Coax, looks like both will come with 15m of coax. Then you need J-Pole to screw to your gutter, and some pig tail connectors to your scanner, you can get that from Jaycar store. or Trade Me. Your coverage will be greatly enhanced
I use to use Diamond D130 but switched to US made Omni-X shipped from US. Very happy with that.