An Australian senator has been slammed for a comment he made on social media less than an hour after a man drove a vehicle into pedestrians in downtown Melbourne.

Senator David Leyonhjelm, who represents New South Wales for the Liberal Democratic Party, shared a story from the Herald Sun about the breaking news.

"It's feared at least one pedestrian has been hit in the Bourke St Mall by a rogue car diving erratically. More soon,'' the newspaper tweeted out.

Leyonhjelm commented on his Twitter page: "Probably one of those semi-automatic assault cars.''

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The off-the-cuff comment has backfired on the politician, with members of the public calling it inappropriate given the incident was still happening.

One woman wrote: "What an appalling thing to say in reaction to this event. Where is your compassion? You are supposed to be a leader.''

Another unimpressed user said: "David Leyonhjelm thinks the Bourke St mall deaths are comedic, what a complete tosser.''