An emergency declaration has been in place for the past three hours in south-west Queensland due to the danger of a toxic substance in the Maranoa region.

Police said emergency services responded to reports two incompatible chemicals had been mixed near the reservoir at Wallumbilla, about 40 kilometres from Roma, about 1pm on Wednesday.

A declaration was made under the Public Safety and Preservation Act at 1.45pm and the boundaries of Burke Street, Russell Street and the western end of High Street have been closed.

Police are asking people to avoid the area or if indoors, close any windows and doors and turn on air conditioning.

Any reports of noxious odours related to the incident should be reported to police.