Maylea and Hadley, who are 8 and 11 years old, have not been seen since they went missing from Ross Creek in the Ballarat area on January 18.
The children are believed to be in New South Wales or Victoria.
They were last seen travelling in a black 2013 Ford Territory.
The license plate number of this vehicle is 1AD7YYH.
Anyone with information about their whereabouts should contact police.