An elderly man and two police officers have been injured after their cars collided in Melbourne's northwest.

The officers were attending the scene of a crash when they collided with another car at St Albans after 10pm (AEDT) on Thursday.

The 74-year-old driver of the other car was taken to the Royal Melbourne Hospital for observation and returned a positive preliminary breath test, police said.

The two male officers suffered minor injuries with one taken to hospital for observation.

An investigation is underway.