Victoria Police said people were making it easy for opportunistic thieves to steal from cars, with more than 100 vehicles broken into every day across Melbourne.

Assistant Commissioner Ken Lay said Casey was a major "theft from motor car'' hot spot.

"Last year, more than 34,000 theft from motor vehicle offences were reported to Victoria Police and this figure has already been exceeded this financial year.''

Senior Sergeant Bernie Kaminski of Casey police said officers were actively targeting the issue but people also needed to take more responsibility and think twice before leaving valuables in their cars.

"If thieves look into your car and see no items of interest, they will simply keep walking,'' he said.

"However, if they spot some valuables it only takes 20 to 30 seconds to break the window, reach in and steal something.

"Nowadays, there are various methods of shattering glass without making noise or alerting passers by, so thefts often go undetected.''

Victoria Police statistics show that almost half of all cars broken into have been accessed by smashing a window.

Since 2001, vehicle theft has cost Victoria more than $103 billion.

The most common items stolen are cash, mobile phones, wallets and laptop computers.

There has also been a dramatic increase in the theft of navigational equipment, or GPS units, with the offences almost tripling.

Senior Sergeant Kaminski said people could take simple steps to protect their cars and belongings, from installing immobilisers to locking windows and doors.