A shed storing vintage steam engines in Ashburton was seriously damaged in a suspicious fire early today.

Fire crews were called to the Plains Vintage Railway and Historical Museum about 5.15am, southern fire communications shift manager Tim Reynolds said.

The shed was "well involved" when crews arrived, he said.

Five appliances and a tanker were sent to the blaze.

Police and fire investigators would arrive at the scene today, Mr Reynolds said.

It was not known how badly damaged the trains were.