A "car bomb" which exploded after being driven into the Australian Christian Lobby's (ACL) headquarters in Canberra overnight has been described by its director as an "attack on free speech", but the government has said it was not a politically-motivated or terror-related attack.

No one was at the office when the gas bottle-filled van careered into the building, which sustained damage to the ground floor and had its windows blasted out, but the ACL's managing director Lyle Shelton said it was a shocking deliberate act against the group.

"This is an attack on free speech in Australia, which I am deeply shocked about and never thought I would see in my lifetime," he told the press on Thursday.

He later told the Australian Broadcasting Corporation (ABC) that an attack on the ACL's headquarters was likely because of the group's public opposition to controversial topic such as same-sex marriage.

"I think something of this nature, that appears to be so deliberate, is an attack against the sort of things that we've been saying in the public square," he said.

"And obviously someone didn't like that which we stand for, which we've advocated publicly."

Deputy Prime Minister Barnaby Joyce has described the incident as a "disgusting attack", but Justice Minister said it was not being treated as a terrorist incident.

"The police will obviously now need to use their investigative techniques to track down what the motivation was for this particular attack," Michael Keenan told the ABC.

"At the moment they are not treating it as a terrorist incident and there's no ongoing safety issues for the Canberra community."

Police said a male driver of the van was the only person injured. He was reportedly taken to Canberra hospital late on Wednesday.

source: Xinhua