Drug free sport advocates say they've been warning athletes for some time now that anabolic steroids could be in supplements.

A study this week revealed at least six supplements on sale in New Zealand contain steroids that are not declared on their labels.

CEO of Drug Free Sport New Zealand Nick Paterson told Mike Hosking he hopes it is accidental.

Mr Paterson said this wasn't a huge surprise to him and athletes had been warned.

"It's the responsibility of the athlete to understand what is going into their body. So the question for athletes is do you know what is going in? Do you need to take a supplement? Is it required? And they've got to give it some really serious thought."

LISTEN TO THE FULL INTERVIEW WITH NICK PATERSON ABOVE