The medium, fast-moving bushfire, 15 kilometres south-east of Goroke, started before 3pm at Karnak-R. Mitchell Road and is travelling in a north-easterly direction.

Incident controller Russell Manning estimated the fire, which had grown to about 600 hectares by 6pm, could reach 1000 hectares before it was contained.

He said authorities were investigating the possibility the fire started from a windmill on private property.

Mr Manning advised Goroke was not in the path of the fire.

He said Horsham was a long way away from the blaze.

"We have responded to about three calls saying Mt Arapiles is on fire, because people can see smoke from Horsham," he said.

Mr Manning encouraged people to call the Victorian Bushfire Information Line on 1800 240 667 for more information.

Authorities expect the fire to impact Gymbowen, Mitre and Tooan between now and 8pm.

The fire is spotting into grassland in the areas of Nhill-Harrow Road, Cyrill-Ampts Road, Morgans Road and Blackers Road.

A relief centre has been established in Horsham Town Hall for people who are relocating.

There is a manned road block on the Nhill-Harrow Road at the Natimuk-Francis Road intersection, and another on the Nhill-Harrow Road at the Wimmera Highway intersection.

* Emergency services have also closed the Western Highway between Ararat and Beaufort because of a fire near Ballarat.

Authorities closed the highway about 5pm.

The out-of-control grass fire is in the Chepstowe area and is about 50 hectares.