A woman sexually assaulted by a man posing as a police officer is too scared to return home, detectives say.

The 36-year-old woman and her 42-year-old partner were tied up when two men armed with a gun burst into their rural Lancefield property, north of Melbourne, about 2am on May 29.

The thieves, who posed as police officers, entered the house through an unlocked door and ransacked the property while searching for money.

One of the men took the woman into a bedroom, where she was threatened and sexually assaulted.

Both men, who were wearing dark balaclavas and yellow fluorescent vests with the word "police" on them, fled in a car.

Detective Senior Sergeant Shane Manuell​ said the attack may have been a case of mistaken identity.

"This was a terrifying experience for the victims, who were attacked in their own home," he told radio station 3AW.

"There's nothing to suggest these people were actively targeted. It may be a case of mistaken identity, or the offenders have been given some wrong information about the property. From what we can gather the people involved are just average everyday citizens in the community of Lancefield, and as a result it's pretty disturbing.

"She's obviously traumatised at what's happened. They're are scared to go back home as a result of this. She's very disturbed and so is her partner."

Detectives from the Sex Crimes Squad released a computer-generated image of a man they hope to question, after making a breakthrough in the case on Tuesday.

The man is described as being about 182 centimetres tall, between 30 and 40 years' old, with a medium build and pale skin.

He was wearing dark tracksuit pants and top, as well as the fake police vest.

Neighbours have reported seeing a pale-coloured, four-door sedan in the area about the time of the home invasion.

They have described the car as being an older model, box-shaped vehicle, possibly from the 1990s. The car had a black trim running horizontally along the doors from the rear mudguard to the front mudguard.

Senior Sergeant Manuell said it was believed the men had a .22 calibre rifle.

"We're concerned that the offenders remain at large, and are doing everything we can to find those responsible," he said.

"We're appealing for anyone who might have seen anything suspicious in the vicinity of Red Gap Road on the night to get in contact with us. Any piece of information, no matter how trivial it might seem, could be the final piece of the jigsaw."

Police will set up a caravan as an information post in Lancefield next week to appeal for further information from the public.

The caravan will be in Lancefield between 9am and 5pm next Thursday and between 10am and 2pm next Friday.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000.