The matter was still active, no charges had been laid and the investigation was unfinished as a result of the driver's "medical condition," an ACT Policing spokesman said.

The spokesman did not elaborate on the condition of the 35-year-old Canberra man.

Australian Christian Lobby managing director Lyle Shelton said that while police had been talking to him, he was frustrated they had not answered his questions.

"I've written to the police seeking more information and am still waiting to hear back from them about how they could so quickly make a judgement call that there was no political motivation for the attack," Mr Shelton said.

"That still perplexes and concerns me because the speed at which they made that judgement based on an interview with a driver that was so severely injured does not seem plausible, and yet the investigation is still ongoing."

He said tenants below the ACL office had vacated the two-storey Deakin office building, which was badly damaged by fire.

Mr Shelton detailed threats the office received in the months leading up to December 21, including emails consisting of graphic same-sex pornography and a string of menacing phone calls and death threats.

He claimed that in late 2016 someone "known as a political activist" suspiciously entered the building and soiled the toilets.

He said police had not informed him of the driver's ongoing medical condition.

Police allege on the evening of December 21, the man drove a van carrying gas cylinders that exploded outside the Deakin office, before walking five kilometres to Canberra Hospital, where he was deemed in a critical condition.

The next night, the driver was flown to the specialist burns unit at Concord Hospital in Sydney, where he was in a serious but stable condition three days later.

Since then, spokespeople for Sydney Local Health District have said the man declined permission for a public update on his condition.

The morning after the suspicious car fire, Mr Shelton was convinced it was politically motivated and linked it to controversy over his organisation's stances on same-sex marriage and the Safe Schools initiative.

Hours later, ACT Policing said in a statement it had established the actions of the driver "were not politically, religiously or ideologically motivated". However, it said its investigation would take in previous threats to the ACL.

Mr Shelton said he still doubted the incident was a random act with no political link, but said he was seeking to "understand clearly the motivation and our ability to asses any future threat to our staff and our property".