One truck was graffitied with the word "false" in spray paint, while a bag of faeces was left on the truck bed of the other truck.

On further inspection, it was also found that faeces had been smeared inside the door handle on the driver's side of the truck.

According to the managing director of Big Impact Advertising, Tamer Bakr, who owns the trucks, the door handle discovery was made when a staff member tried to open the door.

"It's a very low act and shouldn't be in a political arena," Mr Bakr said.

"I started and operated Big Impact for 11 years. We've done many political campaigns for Labor, for Liberals, for corporations, for small business - we've never had such aggressive attacks on the trucks like this, on the billboards."

Mr Bakr said it was particularly egregious as it affected him and his employees, not any politicians.

"It goes to us and we have nothing to do with it, we're just operators."

They were parked in Ainslie, near the Ainslie football club where the Liberal Party held its ACT campaign launch yesterday.

A spokeswoman for the Canberra Liberals' campaign blasted the vandalism.

"Those responsible for this abhorrent behaviour have no respect for their fellow Canberrans, no respect for the small business owner or truck drivers just doing their job."

The trucks, which feature images of Labor leader Bill Shorten and Chief Minister Andrew Barr, feature the words "Labor will tax you to death".

Spray paint was used on the other truck, with someone taking issue with the billboard's message and also spray painted the number plate as well.

The police hadn't been involved, Mr Bakr said, as his priority was getting the trucks back to the depot and cleaned up and on the road again.

He said he planned "to persist with the campaign and the message that we will not be stopped."

The Canberra Liberals are also crying foul over their thwarted attempt to advertise on the billboard at the National Convention Centre.

The party's spokeswoman said the ad was refused because it targeted Chief Minister Andrew Barr, and the National Convention Centre is run by the ACT government.

"These ads have been censored, not because they are political, but because they are directly critical of the Barr government," she said.

A spokeswoman from the National Convention Centre said the decision was made after consulting with the ACT government.

The Chief Minister, Treasury and Economic Development Directorate said neither Mr Barr or his office were involved in the decision.

"Official guidance for activity during the 2019 Federal Government Period states that official resources should not be utilised for political advertising," the spokeswoman said.