Pictures showed that the front of the 1927 building had been damaged, with the ornate front doors completely destroyed.
A spokesman for ACT Policing said: “There has been ongoing protest activity in front of the old Parliament House for the last fourteen days.
“The police investigation into the cause of the fire has begun.
“The old parliament building remains closed for the time being.”
Deputy Prime Minister Barnaby Joyce said that if the fire was lit deliberately, it would be “an absolute disgrace.”
“This is Parliament, which for so long has supported the freedoms we have as a democracy, and which has made the decisions that formed us as a nation that fought to increase the rights and corrections of our colonial past,” he wrote. Mr. Joyce on Facebook.
“If anyone tries to make a statement, it’s a very bad one that will be received with overwhelming disgust.”
Opposition leader Anthony Albanese said it was a “very historic building”.
“I had the great honor of working for Tom Uren in that building. It is used by school groups, it is used for educational purposes as well as for functions,” he said.
“It’s an important building and I just hope there is only minimal damage.”
Former Nationals leader Michael McCormack said there should be “quick and serious” consequences for those responsible.
“How shameful. A monstrous attack on our democracy, our history, our sovereignty. This modern penchant for tearing down our past serves no purpose,” he said on Twitter.
Resource Secretary Keith Pitt also weighed in on Twitter, commenting: “Whatever your case is, it is not the answer to set the old Parliament House on fire and endanger them all … Hope everyone is safe.”
The building ceased to be used as the seat of Parliament in 1988 and is now home to the Museum of Australian Democracy.