Parcel delivery scams that infect computers with malicious software and hold victims' data to ransom are on the rise in the lead-up to Christmas, the federal consumer watchdog has warned.

The scams increasingly target small businesses and threaten to destroy or delete important information from hard drives unless ransoms of up to $1000 are paid.

An email warning a victim they have missed a courier.

The Australian Competition and Consumer Commission said scammers posing as legitimate postal services, including Australia Post and FedEx, send victims emails telling them they have missed a courier. The emails, which appear genuine and may include the victim's name and address, contain instructions about how to retrieve the package.

But the emails contain executable files (.exe) that when downloaded install "ransomware", which infects and locks the computer. Scammers then threaten to delete files off the hard drive if the ransom, typically about $500, is not paid. In one case reported to the ACCC the ransom was doubled to $1000 if it was not paid within 48 hours.