Debris from the fire at Eastown Timber on March 4 has been found to contain asbestos.
Samples were gathered by Whanganui East resident Bridget McArthur around the intersection of Jellicoe and Maketu streets and collected by Horizons Regional Council on March 9.
Whanganui Public Health conducted the testing and will now be leading the response.
Fragments of building paper were found on a property about 400m away, Whanganui medical officer of health Patrick O'Connor said.
"Tests show the presence of asbestos fibres in the paper. This material is red on one side and grey on the other.