They failed to return in some areas after a cold and tough winter dampened their breeding grounds on the South Island.
“We've heard from many monarch lovers in Canterbury and Otago that the monarchs haven't returned this summer ... and it's something that's got us baffled,” says Jacqui Knight, secretary of the Monarch Butterfly N.Z. Trust.
While related to their North American cousins, New Zealand monarchs don’t migrate vast distances.
They instead adapt to local conditions, often wintering along the coast where temperatures seldom fall below 50 degrees.
Experts say climate change may be what has affected the monarchs recently, but they don’t know exactly how.
“Monarchs are an indicator species, telling us a lot about how other insects are going, and this is something to watch closely as we need our insects,” said Knight.
Experts say the colorful insects are likely to rebound in the years to come as colonies recover from last winter's adverse conditions.