A huge swarm of insects over the North Island could be causing a mysterious band of activity on MetService's rain radar.
In a blog post on the MetService website, forecaster Peter Kreft said people looking at radar images between 7pm yesterday and 11.20am today could have been forgiven for thinking there was a lot of light rain over the northern half of the North Island and west of Auckland.
But apart from parts of Gisborne and Hawkes Bay, there was almost no rain over the North Island overnight.
Mr Kreft said they were not sure what was behind the radar image.
"But we strongly suspect the echo is swarms of insects. To show up in radar imagery like this, they must have some of the characteristics of precipitation particles: similar size, fairly numerous, enough water content (or perhaps coating)."
Similar radar echoes had been observed over the North Island, to a greater or lesser extent, on most evenings in February.
Mr Kreft said the weather over the North Island had been settled and very dry during this time.