The New Zealand heavyweight tested positive as part of a drugs test carried out the very day he faced 11th-round loss against English fighter Fabio Wardley.
The British boxer and Parker were part of a pulsating contest staged in the UK capital last October to determine the next mandatory WBO contender against reigning heavyweight king Ukrainian champion Usyk.
The fighter was previously the leading contender to meet the champion after compiling a notable six wins in a row, however put his contender position in jeopardy in the UK and endured an 11th-round stoppage.
A standard screening conducted on Parker by Vada on the day of the bout showed an adverse result and now leaves Parker with a battle to prove his innocence.
Parker's promotional company released an announcement stating: "Vada notified everyone involved regarding Joseph Parker returned an adverse finding subsequent to a doping test conducted on October 25th. During the ongoing inquiry, no additional comment will be made at this time."
Parker became heavyweight champion six years ago before he was defeated for his championship belt by unanimous decision versus Anthony Joshua in Wales two years later.
Parker has been a regular competitor in the UK after that, competing in eight bouts altogether on UK territory and relocating to the Irish capital for training camps alongside esteemed mentor Andy Lee in recent years.
Coached by Andy Lee, the boxer recovered from the 2022 setback against Joe Joyce to achieve multiple wins which put him as the top contender to face Oleksandr Usyk prior to he suffered defeat to Wardley.
Subsequent to Parker's sample showed an adverse finding when he fought Wardley, allegedly for cocaine, a probe about the test outcome has been started.