A van laden with gas cylinders has exploded outside the headquarters of an Australian Christian lobby group, but police on Thursday said the blast was neither politically nor religiously motivated.

Australian Christian Lobby's managing director Lyle Shelton tweeted pictures of the burnt-out white vehicle and said he was "shocked that this could happen in Australia".

He added that the group this year received death threats for taking a stand against same-sex marriage, suggesting the incident could be linked.

But Australian Capital Territory police commander Mark Walters said the motive for the incident, in which only the driver was injured, remained unclear.

"The driver appears to have ignited gas cylinders within the vehicle, causing an explosion which damaged the vehicle and building," he said.

"Police spoke briefly with the man before he continued with treatment. Police were able to establish the man's actions were not politically, religiously or ideologically motivated."

The driver, a 35-year-old Australian, walked to hospital after the blast and was in a critical condition with serious burns. Police plan to speak with him again as soon as they can.