The North Sydney winger took a step towards realising his dream of breaking into the NRL with a four-try haul against Wyong in a NSW Cup trial last Saturday. But his career was on hold just two days later, after he allegedly admitted in a text message to using performance-enhancing substances.

The 23-year-old claims he engaged in what he thought was SMS banter with a Bears teammate, although the exchange is believed to have taken place with the teammate's ex-girlfriend.

The texts were then forwarded to Bears officials and the media, and the matter is now the subject of an investigation by the Australian Sports Anti-Doping Authority.

Johnston, who will meet ASADA officials on Monday, maintains he has never taken drugs, and says the messages were a joke that backfired.

''I'm completely clean, I'm innocent. I said some stupid things that have been blown way out of proportion,'' Johnston told Fairfax Media.

''I woke up on Monday morning and got a text on my phone from this app called Kick. I thought it was one of the boys from my team and I kept talking to them. They were saying 'I'm sore from the weekend still' and 'I've been taking stuff I shouldn't be'. And I went 'Yeah, we all do'. I was joking around, you just agree with your teammates.

''And then they were like 'Can you get me some?' because I know someone who knows about it. So I supplied the number and they go 'Can I call you soon?' I made a few jokes about yabbie pumps … it was just a joke me and some of my mates use. Anyway, I thought it was my mate and then this girl - I still don't know who she is - called up and said: ''You're [busted], you're going down. This is going to [the media].

''It was just a massive joke and then all of a sudden it was completely blown out of proportion.''

It is understood police are looking to question the woman involved in the text exchange.

Johnston led the NSW Cup try-scorers in 2011 with 27, earning him a place in the NSW residents team and a contract with South Sydney.

But the Berowra Wallabies Junior, who has taken five drug tests during his career and never tested positive, is yet to play first grade.

''I've been clean my entire life, I haven't touched anything - I've trained since I was 12,'' Johnston said. ''I'm totally stressed. This is the game I love, that I have played since I was six. I work around football, everything I've done is to make me a better footballer. I've trained my arse off for so many years, and now I feel it's taken more than it's given me.

''It's just painful. The effect it's had on the family - I've never seen my dad cry before. I feel bad about it because I might lose football. Seeing him like that, because he's worried about me, hurts me. My mum is coming into my room every morning asking if I'm OK. Everyone is shocked by what's happened because of this massive joke.''

Johnston's father, Col, fears his son has unfairly become the public face of the ACC's crackdown on sport's doping cheats. ''There's been a lot of pressure coming out of the press with the inquiries,'' Col Johnston said. ''I just hope he hasn't become the face of this.''