The 33-year-old New Zealander suffered an 11th-round stoppage defeat by British boxer Wardley, losing his status as WBO mandatory challenger for undisputed heavyweight champion Oleksandr Usyk.
"The Voluntary Anti-Doping Association (VADA) last night informed all required parties that Joseph Parker returned an adverse finding following an anti-doping test conducted on 25 October in relation to his bout with Fabio Wardley," promoters Queensberry said in a statement.
"While the matter is investigated further, no additional comment will be made at this time."
Parker had put himself on a potential fight course against the outstanding Usyk following six successive wins, a sequence that included victories over Deontay Wilder, Zhilei Zhang and Martin Bakole.
However, he could now face a ban depending on the outcome of the investigation.
Parker became world champion in 2016, but he lost his WBO belt by unanimous decision to Anthony Joshua in Cardiff two years later.