The 29-year-old Mullen's career appears all but over if a report that his B-sample has also tested positive to steroid Drostanolone is correct, leaving him facing a maximum four-year ban.

The Australian Sports Anti-Doping Authority has yet to confirm the B test result with either the NRL or Newcastle, but Channel Seven reported the sample returned positive to a banned substance.

At this point, Mullen remains suspended provisionally by the governing body after an out-of-season test by ASADA last November came back positive

"Anyone who loses a career, it's disappointing for them, and ultimately disappointing for the game. But there are always consequences for poor decisions," Greenberg said on Friday.

Newcastle CEO Matt Gidley said the club was still waiting to hear from ASADA about the B-sample outcome with the new NRL season just three weeks away.

"It's difficult. I've spoken to ASADA about that. It's difficult for them to give us any definite time frame," Gidley said.

"They understand we would like this to move as quickly as practically possible … we're hopeful it's a matter of weeks rather than months."

Mullen’s manager, Steve Gillis, told the Herald he had “nothing to report” and there would be “no comments in the forseeable future”.

Greenberg reiterated penalties were clear and well-known.

"We do a huge amount of education and players need to make good choices and when they don't and they take risks, there are massive consequences," he said.

"We're talking about career-defining consequences. I think all players are acutely aware of their responsibilities."

It is understood ASADA is still investigating the background to Mullen's positive test.

"When an athlete returns an Adverse Analytical Finding, we fully explore the circumstances around that AAF," ASADA said. "How the athlete accessed the substance, the role of the people surrounding the athlete and any other issues pertaining to the use of the substance by the athlete--in doing so, we pursue and follow any leads as part of that process."

Gidley said there was no suggestion that any other player could be caught up in the case.

"There's been no indication that there is anyone else who has been implicated in this," he said.

"At the moment Jarrod has returned a positive test, that's unfortunate but at this stage there is no one else under investigation."