The committee backdated the two-year suspension by 12 months to October 9, 2017, to account for the delay in proceedings and Johnston's timely admission which means he will miss only this season.
Documents from the hearing stated Johnston purchased 20 millilitres of Clenbuterol on or about September 22, 2014, from NZ Clenbuterol.
Johnston claimed it was purchased for use by his wife in an attempt to reduce weight.
Clenbuterol is used for this purpose but the Johnstons provided statements to indicate Clenbuterol wasn't used because of the state it was in when it arrived.
The Tauranga Boys' College old boy has been one of the most dominant players in Hawke's Bay's club rugby competition for the past three years and was considered unlucky to miss out on the Magpies Mitre 10 Cup squad last year.
He won the 2018 McDonald's-Hawke's Bay Today Club Rugby Player of the Year award by six points from fellow loosie Solomone Funaki from the MAC club.
During the buildup to last year's Maddison Trophy semifinals long-serving NOBM head coach Craig Gowler described Johnston as the most inspirational footy player he had seen for a long time.
Johnston has helped NOBM win three Nash Cup titles and two Maddison Trophy titles after joining the club in 2015 following one season with Napier Technical Old Boys in 2014 after moving to the Bay from Waikato.
Johnston was unavailable for comment today as he and wife Alice were in hospital awaiting the arrival of their first child.
"NOBM are aware of the decision issued by the NZR Judicial Committee in relation to Chris Johnston, and respects the Judicial Committee's decision," NOBM club chairman Terry Gittings said.
"NOBM are fully committed to providing whatever support it can to Chris and his family during this trying time for him," Gittings added.
"Chris is a well-liked member of our rugby community. However, Chris has made a huge mistake in this instance and he is being punished accordingly. Chris has accepted his sanction and will therefore sit out all rugby in 2019," Hawke's Bay union CEO Jay Campbell said.
"This sanction is a timely reminder that all players need to be vigilant when purchasing substances online and to be aware that the anti-doping rules apply to all registered players not just those playing at the top end,'' Campbell added.
Roff was a registered club player in Southland when he purchased Clenbuterol in December 2014.
Roff admitted the possession and use (or attempted use) of the prohibited substance but told the committee he purchased it in the hope it would assist with his asthma condition.
The committee imposed to a two-year suspension, backdated six months to May 29, 2018, to take account of delays in the allegations being heard.
While Roff did admit the allegations, he was not entitled to further backdating under the rules since he had played at least one match (in Scotland) after being confronted with the allegations.