Four pro-Palestine protestors have been charged with Commonwealth Trespass after climbing on the roof of Canberra's Parliament House on Thursday.

The activists unfurled a large black banner with the slogan in white writing "from the river to the sea, Palestine will be free" and "no peace on stolen land", as another hung under the Australian Coat of Arms read "war crimes enabled here".

Officers were spotted at the front of Parliament House monitoring the protest as one activist read out a speech from his phone.

The protesters, three men and one woman, were charged at court on Thursday afternoon and have been released on bail.

The maximum penalty for their crime is a fine of $2,750.

The demonstration was decried by Deputy Prime Minister Richard Marles on Thursday afternoon, telling Sky News' Tom Connell while freedom of speech was critical in Australia the defacement of public property was "disrespectful".

"It's very important that as people express their views they do so in a way which is peaceful, which does not put lives at risk, which does not engage in vandalism and we've seen quite a bit of vandalism of public property and public institutions over the last few days and weeks and months actually," he said.

"To do that, really, is to act in a way which is disrespectful to our fellow Australians."

Alongside the banners, the demonstrators made about half a dozen paper aeroplanes, which had slogans written on them, and threw them from the roof to the media.

Nationals MP Barnaby Joyce said there were "serious questions" to answer about how the incident occurred.

"We should have an inquiry. Find out exactly who... you should get them down and go through them like a packet of salts and find out everything about them," he said to Sky News Australia host Kieran Gilbert on News Day.

"This was planned. How did they plan it?"

Liberal Senator James Paterson echoed the remarks of Mr Joyce, adding that tens of millions of dollars had been spent to fortify and protect Parliament House.

"If someone can get access to the roof, then their access to the building is much easier than it otherwise would be. So it's very serious that these people have been able to do so and the security mechanisms in this building have failed to prevent it," he said.

"These are protesters, they don't appear to have any violent intent. But if protesters can do this, then someone with violent intent could also do this."

The protesters took their banners off the building about 11:30am Thursday.