According to court documents seen by APNZ, the ring involved Musson picking up packages of steroids from mail depots using a fake driving licence as ID. The contents were described as 'paint restoring wax', 'acrylic nail sealing wax' and 'travel pack cosmetics'.
Musson then returned to his Christchurch property and packaged the medicines, which came in in liquid and powder form, into vials and tablets using a makeshift laboratory in his garage.
Ministry of Health officials said the process fell "well short of good manufacturing practice", raising concerns over health risks for users.
Musson, crowned Mr New Zealand Athletic 2005, also used his own pill press to make tablets, which he would dye, and print with his own brand, under 'Phoenix' or other names.
He used a storage facility rented in his mother-in-law's name to store his cache of medicines and raw ingredients.
Medicines with a street value of $300,000 were seized at his premise when he was raided last May.
A total of 17 consignments of imported raw ingredients with a street value of around $641,000 were imported by Musson and intercepted by Customs and the Ministry of Health.
"The medicines were distributed throughout New Zealand on a large scale," the court papers say.
He used a number of distributors through the country.
"It was a sophisticated ring, which Musson was seeking to expand," state the documents.
It was a lucrative business, with the professional personal trainer operating on margins of between 2000 and 5000 per cent for home-made products.
His margin on imported products was the much smaller but still substantial 290 per cent.
The court papers say the substances Musson sold are banned for the protection of athletes' health and to protect medical and sports ethics.
"Banned substances are considered to have a corrosive effect on competitive sport," the documents state.
Yesterday Musson appeared at Christchurch District Court and pleaded guilty to 156 charges laid under the Medicines Act 1981.
He also pleaded guilty to two police charges of forging a driver's licence and using that fake document.
Another 155 charges were withdrawn after defence counsel Pip Hall reached an agreement with Ministry of Health lawyer Sarah Jamieson.
Musson, of Bishopdale, Christchurch, appeared in court from custody after police alleged he had been offending while on bail. He will remain behind bars until he is sentenced on June 27.
Judge Brian Callaghan called for pre-sentence reports including one which assesses Musson's suitability for home detention.
An application for electronically-monitored bail has been set down for April 26.
• Stunted growth for users aged under 21
• Virilisation, or masculinisation, of women
• Renal failure
• Worsening of heart failure
• Hypertension
• Liver tumours or liver toxicity
• Abnormal cholesterol levels
• Psychotic disturbances
Source: Ministry of Health
