The yellow admiral or kahukōwhai (yellow cloak) was one of the first insects from New Zealand to be formally named and described after being collected on Cook’s 1769-70 voyage.
The yellow admiral bears a superficial resemblance to the red admiral, but unsurprisingly it’s only the yellow admiral that has the distinct yellow patches on the upper wing surfaces. The butterflies are most commonly seen in the summer months but may also overwinter in warm, sheltered places. They have a basking behaviour where they will orient themselves to maximise exposure to the warmth of the sun’s rays.
Caterpillars feed on nettles, and a few nettles growing where people won’t easily blunder into them along with flowering plants for the adults to take nectar from will encourage these butterflies to visit your garden. These butterflies are found throughout much of New Zealand at low altitudes. This species is also found in Australia, Norfolk and Lord Howe Islands. They are strong fliers, and it’s likely that yellow admirals cross the Tasman Sea from Australia reasonably regularly.
Check these links for more info on the Yellow Admiral Butterfly in NZ
iNaturalist.nz: Observations of Vanessa itea
Wikipedia: Information on Vanessa itea
Moths & Butterflies of NZ Trust: Yellow Admiral, Kahukōwhai