Mullen will complete a four-year ASADA ban on Sunday after testing positive late in 2016 to a banned steroid, which he said a physiotherapist had described as an "amino acid" that would help a chronic hamstring injury heal.

Sacked by the Knights soon afterwards, Mullen's life spiralled out of control until he almost died two years ago after overdosing on cocaine and prescription drugs.

He underwent rehabilitation and says he has not touched drugs or a drop of alcohol since, but his fall from grace was not complete until he was sentenced to 300 hours of community service last year after pleading guilty to cocaine supply.

Now the former NSW Origin playmaker, who appeared in 211 NRL games for the Knights and captained them to two play-off victories in 2013, is intent on launching a remarkable comeback.

Turning 34 in April, he will play in the Queensland Cup with the Falcons this year and must serve a 12-month "probation" period before NRL officials will consider allowing him to play in the premier competition.

But rest assured that Mullen's ambition is to eventually return to the NRL, and he has been training intensively to give himself the best possible chance.

"At this stage, I've signed with the Falcons and I'll be playing for 12 months in the Q-Cup," Mullen said.

"The NRL have their rules and I'm happy to abide by them, but if something happens to change that, I'd grab it with both hands.

"At the moment, I'm just focused on the Falcons and I'm really grateful for the opportunity ... but I'd be lying if I said my goal wasn't to get back there [to the NRL].

"If I'm playing good enough football up here in the Q-Cup, maybe some NRL teams will have a look at me.

"I'm under no illusions. Four years out of the game is a long time and it's going to be hard.

"But I've trained really hard to condition myself and I'm confident with how the body has been holding up.

"I'm going to leave nothing in the tank, put it that away."

During his 12 seasons at the Knights, Mullen was always one of their fittest athletes and best trainers, and he feels in good shape after countless field and gym sessions, often before the crack of dawn.

"I feel like I've ticked every box, but match fitness is different," he said.

"You can run around a field as many times as you like, but it's different when you're making tackles or getting tackled, getting up off the ground and dealing with the fatigue.

"So I'm really excited about being able to train with a team again and getting myself prepared to play some games."

He said the opportunity came about by chance.

During a holiday on the Sunshine Coast some time ago, he bumped into Melbourne Storm talent scout and recruitment manager Paul Bunn.

They stayed in touch and eventually Mullen asked his manager, Steve Gillis, to reach out.

Bunn helped broker a deal with the Falcons, for whom the Storm's fringe first-graders play when they are not required for NRL duty.

Mullen said he has had no contact with Storm coach Craig Bellamy. That could change if his performances in the Queensland Cup catch the eye.

He is staying with long-time family friends on the Sunshine Coast and is hoping to find a job to fit in around his football commitments.

He said the toughest part of relocating was leaving his partner, Tamara, and nine-month-old daughter Stevie in Wollongong, but he is hoping they will soon join him north of the border.

Mullen said he had been given a "second chance at life" and was determined to make amends, even completing a TAFE course in alcohol-and-drug counselling, in the hope that he can help others avoid the same mistakes he made.

"Since December 2, 2018, I haven't had a drop of alcohol, or anything else," he said.

"I've really turned my life around and come to terms that I can't go down that path any more."

Falcons CEO Chris Flannery told AAP that Mullen "looks a million dollars" in terms of his physical condition.

"We've done a very thorough due diligence on him, spoke to a lot of people and then obviously spoke to him a fair bit ... [but] it's a long time to be out of the game and we're all going to find out fairly soon just where he's at," Flannery said.