"There's seemingly now a much broader part of the community that is interested in getting bigger and looking better. And that's where steroids work."
Steroids were legal to use, but not to import or prescribe, Mr Steel said.
A gym had approached Drug Free Sport to express its concern about the increase in steroid use. It had developed a code of conduct that it hoped other gyms would follow.
Mr Steel said sportspeople were sharing gyms with steroid users such as body-builders, and could be drawn into substance abuse.
"The danger for us is that [body-building] brings more steroids into the country, into the market, and athletes are in those same gymnasia and there's a potential that they may deliberately or not deliberately get involved with that.
"[Body-builders] are probably the heaviest users and therefore part of the distribution network for those drugs. If they are using, then, as with many other drugs, it may be the next step is them selling in the gym alongside rugby players and athletes and cyclists. There is a potential for that to flow," Mr Steel said.
Customs confirmed that seizures of steroids and gamma-aminobutyric acid (Gaba, used for toning muscles) had increased at the border.
In 2011, 13.5kg of steroids and 63kg of Gaba were seized, compared with just 190 grams of steroids and 1.1kg of Gaba in 2008.
Gym operators told the Herald that steroid-users were mostly confined to body-building or weight-lifting facilities, but the drug was sometimes found in mainstream fitness clubs.
Club Physical chief executive Paul Richards said he was forced to ban a husband and wife from his Te Atatu branch because they were openly dealing in steroids. "They seem to go from gym to gym and get banned from each one. It's short-term thinking; all they want to do is get big. When you're in your 20s, you're not thinking long-term. It's all ego and vanity without any sort of future," he said.
NZ Muscle Nutrition and Bodybuilding general manager Gavin Makins said it was difficult to detect steroid use because it was mostly underground.
Dr Nigel Harris, a senior lecturer in sports and exercise science at the Auckland University of Technology, said: "When you are dealing with any drug that is designed to interfere with the body's natural hormonal processes, you set off a veritable cascade of hormonal events, and a lot of those aren't even known."
Dark power
* Anabolic steroids influence the body's natural hormonal processes.
* Bodybuilders, sportspeople and athletes take them to lift performance and physical appearance.
* Side-effects include shrinking of the testicles, severe acne and aggression or "roid rage".
* Steroid abuse also implicated in cardiovascular disease, liver damage, infertility and loss of libido and mental illness. In some cases users have died.