Canberra Raiders and Australian rugby league star Joel Monaghan is under investigation after a photo emerged showing him in a compromising position with a dog.

Monaghan described the incident as a "simulated act" meant to be a prank.

"Joel can't blame anyone but himself for an act of stupidity that will haunt him for the rest of his life," manager Jim Banaghan said in a statement.

"Joel wants to make it clear that he was the one playing a prank on an absent team mate by simulating the act.

"There are no words of explanation that can be offered because none can be appropriate.

"Joel has to now face his family as well as fans and supporters with that shame and has already undergone counselling to help him cope with the consequences of what has happened."

Raiders Chief Executive Officer Don Furner said an investigation was underway.

"If he did it, it is something he will live with for the rest of his life," Furner told a media conference.

"We don't know whether it's authentic but I am as appalled as everyone else.

"If he did do it, I don't know why he would do it."

The incident reportedly happened during the Raiders' Mad Monday celebrations following their finals loss to Wests Tigers.

The NRL says it will await the outcome of the Raiders investigation.

The photo was reportedly posted on the internet on Monday before being taken down by the Melbourne radio station that had put it online.

The action was too late with the image being re-posted through Twitter.

Banaghan attacked those who had sent the image through cyberspace.

"The fact that someone has sought to compound the situation further by the use of social media only adds to the trauma," he said.

Banaghan added Monaghan was not "in a fit state" emotionally following the release of the image and would not be able to immediately meet with the club.

"He apologises unreservedly for the outrage that people feel at the moment and blames nobody but himself," he said.

"Joel is a genuinely good person who is simply shattered by a moment of sheer madness."

Animal welfare groups were outraged, saying if the image was accurate it was very worrying.

"The guy needs help," an Animal Liberation ACT spokesman told AAP.

Canberra's RSPCA chief Michael Linke said the association was "disgusted and sickened".

"It is undignified and it's completely disrespectful to an animal let alone a home pet," he said.

Mr Linke called on the territory's Labor government to immediately introduce laws that would make the alleged behaviour illegal.

"No laws have been broken and that's the sad thing about this."

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