The elderly couple in Melbourne's south-west who accidentally received a parcel full of methylamphetamine were unaware "of the significance of their find".

The pair immediately contacted police on discovering the alleged 20kg of MDMA with an estimated street value of $10 million at their Hoppers Crossing home on Wednesday afternoon.

The parcel was sent through a courier service.

Senior Sergeant Matthew Kershaw says the drugs are believed to have been sent from overseas, but somewhere along the line a mistake was made.

"$10 million worth of drugs sent to wrong address to an elderly couple - that's quite incredible to comprehend that someone could be that sloppy," he told reporters.

Senior Sergeant Matthew Kershaw speaking to reporters in Werribee. Credit: 7NEWS

Kershaw said the couple could not comprehend the significance of the find as they are quite elderly.

"We're ensuring them they’ve done absolutely nothing wrong and their welfare is paramount," he said.

Detectives executed a warrant at another address in Bundoora after a further 20kg of methylamphetamine was found, also with an estimated street value of $10 million.

A 21-year-old Bundoora man was arrested and charged.

He is due to front court on Thursday afternoon.

Methylamphetamine is most commonly found in Australia in a powder form.

Watch the clip above.