An Eastern Bay butterfly expert believes the colourful insects are the key to unlocking facts about climate change.

Jacqui Knight of the Monarch Butterfly Trust will demonstrate how to tag butterflies at a public meeting tonight in Whakatane.

She says more monarchs need to be tagged to give strong baseline data.

Ms Knight says with this data, they can monitor how they are faring, and how they're affected by climate change.

She says numbers throughout New Zealand have been very low over the summer, which could be a result of the cooler winter last year, and dry summer which preceeded it.