Emergency services were called to the showground in Sydney's west after the driver lost control of the vehicle, causing it to swerve into a fence and directly into the path of onlookers.

Scott Anderson, organiser of the Monster Slam event at Penrith showground, told News Limited that a "full report would be conducted into the incident".

"The truck didn't make contact with any of the spectators," Mr Anderson said.

"We have very strict protocols in place and trucks can be turned off remotely if they need to be.

"Safety is our number one priority", Mr Anderson said.

However, a string of witnesses disputed Mr Anderson's claim that the event was safe.

Todd Maiden, who left before the accident occurred, said he had never seen such a disregard for safety.

"In 20 years of motor sport experience I have never seen such disrespect for safety," he said on Facebook.

"As soon as it started I wanted to leave for the safety of my family. Lucky for them it wasn't a lot bigger".

Kirstie Smith, who witnessed the accident, echoed Mr Maiden's account of the event, describing it as "bad for safety" in a Facebook post.

"I was shocked to hear the people say over the pa hold on to your kids while the utes are driving around the full track which half the track had no fencing around it at all", she wrote.

Organisers billed the event as "an afternoon of adrenaline packed excitement" for fans of the world's best "methanol breathing Monster Trucks".

"Fans will be kept on the edge of their seats with monster trucks reaching new heights with massive airtime, gigantic jumps and electrifying freestyle.

"The adrenaline is sure to be pumping with the monster trucks firing up their engines with a show to excite all ages", the event organisers wrote on social media.

Australian drivers had been competing against monster truck competitors from America.

Police said they could not confirm whether an investigation into the incident would occur.

Event organisers Monster Slam have been contacted for comment.