Police investigating the suspicious disappearance of Melbourne mother Karen Ristevski have broadened their search to country Victoria.

Missing persons squad detectives are focusing their efforts on a "rural area" north-west of Melbourne, acting Sergeant Melissa Seach said on Thursday.

"Investigators are appealing for anyone that resides in this area to consider checking their properties for any changes or any suspicious activity since the disappearance of Karen," Sergeant Search said.

Police would not disclose a specific location, but the focus is on country areas, rather than the outer suburbs of the city.