Two armed burglars tied up and sexually assaulted a woman during a terrifying home invasion northwest of Melbourne on Friday, police say.

Police say two men armed with a gun, disguised as police officers entered the house on Red Gap Road, Lancefield, through an unlocked door about 2am.

The pair woke a 42-year-old man and a 36-year-old woman, tied them up and searched the house for money, a Victoria Police spokeswoman said.

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The spokeswoman said the burglars took the woman into a bedroom, then threatened and sexually assaulted her.

Sexual crime squad Detective Shane Manuel said the burglars were demanding money which the victims did not have.

"We believe at this early stage that this may have been a case of mistaken identity or an instance where the offenders have been given the wrong information about this address," Sergeant Manuel said.

"This would have been a terrifying experience for these people and we are appealing for information from the public to help us catch these offenders."

The offenders were described as wearing fluorescent vests with the world 'police' on them and dark balaclavas that covered the bottom halves of their faces.

Police have asked for anyone with information to contact Crime Stoppers on 1800 333 000 or visit www.crimestoppersvic.com.au.