Three separate fires lit within minutes under a busy stretch of a central Auckland motorway are being treated as suspicious.

The alarm was raised at 5.16am when the fires were spotted burning under the Hopetoun Bridge on Howe St in the heart of the major Spaghetti Junction interchange.

Fire and Emergency northern shift manager Colin Underdown said the fires were fuelled by rubbish.

They quickly grew big enough to be visible to passing motorway traffic.

He said they were quickly extinguished but were being treated as suspicious.

Police had been called in, he said.

Travel on Auckland’s motorway was not affected.