A man and a woman are in custody after driving their stolen car into a police vehicle in Melbourne's north, the fourth such ramming in the area this week.

The 25-year-old Doncaster man is alleged to have been hiding a firearm in the boot of a stolen car at the time.

Police said the police van approached the stolen car on Peterson Avenue in Coburg North about 4:50am when it crashed into them and three other cars before speeding away.

The man and a 25-year-old Hurstbridge woman were arrested in Reservoir a short time later.

Since Sunday there have been four incidents of police cars being rammed by suspected car thieves and armed robbers in the northern suburbs.

A police car was also rammed near Geelong just after 4:00am.

Police said a man fled after he hit their car on Portarlington Road, Drysdale.

Police concerned about impacts

Replacement cars are used when damaged police vehicles are being fixed.

Often the substitute cars do not have lights and sirens, and some police are concerned it limits their ability to do their job.

"We are running out of cars fast" one officer told the ABC.

On average a police car is rammed every three days, with rammings tripling in the past 12 months.

Assistant Commissioner Doug Fryer said the operational response was not being put at risk, as police cars were only out of service for a short time.

"Victoria Police have in excess of 2,800 police cars across the state, that is a significant number," he said.