Peter Gallen, the younger brother of Cronulla NRL captain Paul, is serving a two-year ban for doping.
Gallen was suspended on June 5 last year after returning a positive A-sample doping test while playing with Rockhampton-based Queensland Cup side Central Queensland Capras.
The Australian Sports Anti-Doping Authority (ASADA) made details of the ban public on Friday upon completion of an appeals process.
Gallen, 26, tested positive for dimethylpentylamine, a performance-enhancing substance sometimes found in body-building supplements.
His suspension ends mid-way through 2018.
Capras chief executive Dominique McGregor said Gallen was adamant his taking of the substance was inadvertent.
"All he told us was he was taking a pre-workout powder that you buy from the supplement shops," McGregor told AAP.
"He didn't realise. He said that he'd put it all through the app and nothing came up there."
McGregor said Gallen was stood down by the club after first testing positive in June 2016.
"At the time of the initial testing, he was disappointed and that's why he went for the B (sample) testing, because he thought it may be wrong," McGregor said.
"We've never actually been notified as a club by ASADA about any of this. He notified us as soon as he'd got his initial test back, that it had come back positive.
"He was very disappointed in himself and he agreed with the club that he'd step down immediately, as well, while we wait for the B test to come in."
McGregor otherwise praised Gallen's contribution to the club.
Queensland Rugby League said it would continue rigorous drug testing.
"While it is disappointing to learn of this breach, we are pleased to share that we invest more than any other state sporting organisation in Queensland on our testing and education," a QRL spokesperson said in a statement.
"We will continue to work in with the NRL's whole-of-game testing procedures and we make no apologies for having a rigorous program in place."
Paul Gallen served what was effectively a three-match NRL ban before the end of the 2014 season after he admitted to unknowingly and unwittingly using banned performing enhancing peptides.
The former NSW Origin captain and Kangaroos forward was one of 17 players from the Sharks' 2011 squad to receive 12-month back-dated bans for taking prohibited substances.