Police officer Michael Hirst, who was fatally shot by a gunman in the Melbourne CBD on Sunday night, was one of two Australians killed in a suspected terrorist attack.

Police have said the attack was carried out by two Australians.

Hirst was killed by an unidentified man wearing a mask during a standoff with police in the central business district of the city.

It is understood the suspect was armed with an assault rifle.

The manhunt is continuing for the gunman who is believed to be in his 20s or 30s.

The siege began shortly before midnight on Sunday when police stormed a flat in the area and used a bomb robot to bring down the front door.

There were no reports of injuries to police or the suspect.

Police are seeking two men aged in their 20s, aged in the 40s and 50s, with a history of mental health issues, as well as a woman aged in her 20s.

An armed man wearing an orange jumpsuit was later arrested in the CBD and is being questioned about the attack.

A police spokesperson said police believed the man was in a confrontation with police when he opened fire on the officers.

He was taken to hospital and died of his injuries.

The ABC has sought comment from the federal government.

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