An elderly Melbourne couple has received an unforgettable surprise package in the mail – $10 million worth of methamphetamine.

Police said the couple, from Hoppers Crossing in the city's south-west, signed for the mysterious, and heavy, parcel about 1pm on Wednesday.

Understandably curious, they opened it to find 20 kilograms of the potentially deadly recreational drug, also known as ice.

"[They] had a look inside, observed a white substance in bags and called police straight away," said Senior Sergeant Matthew Kershaw.