There are many hungry monarch caterpillars out there thanks to the warm weather.

But because of a cold, wet winter their favourite snack is in short supply and it's up to Kiwis to get planting.

The winter meant that garden critters like slugs and snails had flourished and had subsequently gobbled up many swan plants, one of few plants the monarch would eat, Monarch Butterfly New Zealand Trust spokeswoman Jacqui Knight said.

"We went into the spring needing lots more swan plants."

In her garden the monarchs had been tucking into swan plants, there was an abundance of food for them, but that was not the case for the rest of the country.

"I probably had about 20 big ones in my garden this time last year and this time I have about three or four," Knight said.

The monarch is a milkweed butterfly and will only eat milkweed plants, like the swan plant.

She said there were people going out to buy plants every day, instead of planting them.

Knight said people did not understand there was a life cycle for everything and it all worked together.

"They think that monarch butterflies are wonderful and nothing else is."

In 2017 the monarch butterflies were seen out of season, brightening up the winter skies of Auckland.

Apart from the Auckland sightings, the trust was aware of overwintering of butterflies in Napier, Christchurch and Nelson.

Knight said monarch butterflies were an indicator species and if we studied how they were changing "we can reflect on how our environment is changing and adjust our behaviour to improve our environment".