To get the best answer to this one, we went straight to the experts. Jacqui Knight from the Moths and Butterflies of New Zealand Trust kindly shared the following advice:

Monarch butterflies need to eat milkweed and there are 3-4 varieties in NZ. There are the swan plants: Gomphocarpus fruticosus and G. physocarpus (the swan plant and giant swan plant from Africa) and Asclepias curassavica with its colourful flowers, from North America.

Caterpillars will eat the leaves, stems, flowers and seed pods on the plants, but they do eat a lot. It is very important that they eat milkweed as they convert chemical compounds in the leaves, protecting the caterpillars as they taste bad to birds. This is their natural defence against most predators.

People have tried feeding them alternatives, which we know affect their physical and reproductive health - so why bother? Instead, the Moths and Butterflies of NZ Trust has set up a Facebook page called "Monarch Matchmaker" where people can network with neighbours if they have too many caterpillars or too much milkweed. There is more information on our website, www.nzbutterflies.org.nz