Wendell Sailor has been banned from playing for an indefinite period after returning a positive drugs test.
Sailor has already been banned this season for a drunken incident
The Wallabies wing, 31, was stood down by the Australian Rugby Union before his New South Wales team's Super 14 match with the Hurricanes on Saturday.
Sailor was notified last week by the Australian Sports Anti-Doping Authority that part A of a sample he provided recently had returned positive.
He could face a two-year ban if a B sample confirms the banned substance.
It is not known yet if it is a recreational or performance-enhancing drug.
But it could effectively finish the career of the high-profile former rugby league international who has a reported A$750,000 (£305,000)-a-year contract with the ARU.
"We have approached the Rugby Union Players' Association to request legal advice," Sailor's manager, Greg Keenan, said in a statement.
"At this stage, Wendell has no comment to make."
The NSW Waratahs had earlier said Sailor had been stood down for an alleged breach of the ARU's code of conduct, but made no reference to the positive test.
NSWRU chief executive Fraser Neill said he was unable to discuss the case because of legal reasons, but said it was one NSW and the ARU viewed "most seriously".
The ARU later issued a statement confirming what Sailor's manager had already disclosed.
"It is important to stress that the ARU is not in a position to respond to hypothetical questions about what might happen if Sailor's 'B' sample is positive or if he chooses to accept his 'A' sample," it said.
"The ARU condemns the use of performance-enhancing drugs
and the use of any other drugs that are on WADA's prohibited
Sailor switched codes in a lucrative deal in 2002, and had four seasons with the Queensland Reds before moving to the Sydney-based Waratahs this year on a two-year agreement.
He has already been fined and suspended for three matches this season over a drunken nightclub incident in Cape Town.
He was initially suspended for one match, fined A$500 and sent home from South Africa by the Waratahs.
But he had a suspended ARU two-match ban hanging over him from another indiscretion with the Wallabies in the same South African city last year, and was fined an extra A$3500.
Sailor's New South Wales team missed out on a home Super 14 semi-final when they slipped to a 19-14 defeat against the Hurricanes, who they will now face again in Wellington in next weekend's semis.