Eight hundred litres of pesticide spray Roundup was destroyed by flames on Thursday, with a further 750 litres of the chemical damaged.

Country Fire Authority operations officer Chris Eagle said Roundup fumes would have dissipated on contact with air, removing the risk of paddock or dam contamination.

Murray Goulburn staff would need to assess smoke-affected goods inside the farm supplies business to determine whether they could be salvaged, he said.

The blaze started inside the store's office, later spreading to chemical storage areas.

Koroit residents were warned via text message and phone calls about 11.30pm on Thursday not to go near the Murray Goulburn store, to stay inside their homes and to ensure all their windows were shut.

Joanne Archbold told The Standard yesterday that she had experienced a burning sensation in her nose and throat on Thursday night, having sat outside her home to watch the response of fire authorities.

"We need to have a full list of what was burnt up there," she said.

"It is very concerning. The chemicals did cause (medical) symptoms."

Ms Archbold said she felt that more should have been done to alert the public earlier of the need to remain indoors and avoid breathing in the smoke.