Police have said it is "extremely lucky" nobody was killed when an improvised explosive device launched from a vacant block of land tore through the roof of a nearby home in south-east Melbourne.

Key points: Two people who were in the house as the pipe smashed through their ceiling were not physically injured

Two people who were in the house as the pipe smashed through their ceiling were not physically injured Police are trying to identify a young man and woman captured on CCTV footage before the explosion

Police are trying to identify a young man and woman captured on CCTV footage before the explosion Those responsible could face serious criminal charges

Detective Senior Constable Heath Lyon said about 15 seconds passed between the sound of an explosion at Carrum Downs and the moment a projectile built from a 7-kilogram metal tube crashed through the ceiling of a Hillview Drive home.

"The occupants stated the son is usually home from work at that time and would be normally showering, so he could have very easily been killed or seriously injured," Senior Constable Lyon said.

He said the two occupants who were inside the house at the time were not physically injured.

"[It is] extremely lucky no-one was killed … that time of the day there's people walking after school, walking their dogs."

Senior Constable Lyon said it was "a rather large explosion". ( Supplied: Victoria Police )

The Arson and Explosives Squad has appealed for the public's help in identifying a young man and woman, who were seen walking away from the vacant block shortly before the explosion about 5:00pm on August 28.

Senior Constable Lyon said he was unsure what might have motivated someone to launch the potentially lethal missile.

"Intentions can vary from just skylarking to more of a serious nature of intentionally firing it at someone or a property and once again until we speak to these people I won't know what their intentions were," he said.

Police released CCTV footage of the pair they believed could help them with their investigation. ( Supplied: Victoria Police )

The man was described as Caucasian in appearance, about 180 centimetres tall and with a medium build.

The woman was also described as Caucasian in appearance with a medium build and long hair tied in a ponytail.

The Hillview Drive home was 150 metres away from where the projectile was launched, police said. ( Supplied: Victoria Police )

Senior Constable Lyon said those responsible for launching the projectile would face serious charges which could include possessing an explosive substance and conduct endangering a life.

When asked about reports that similar loud bangs had been heard in the area recently, he said police were open to inquiries that there could be a second incident.

Police urged anyone with information to contact Crime Stoppers.