"The appointment was completely insensitive and reflected a lack of judgement," deputy general secretary Gideon du Plessis said in a statement.
"It also confirmed Solidarity's belief that the Aurora directors seemingly have political indemnity."
Hulley was one of the directors of the controversial Aurora mine company along with Zondwa Mandela, grandson of former president Nelson Mandela, and Khulubuse Zuma, President Jacob Zuma's nephew.
Aurora, which was singled out as the preferred bidder for Pamodzi's mines when they went into liquidation in 2009, had its management contract cancelled and vacated the premises.
The company had been accused of the destruction of infrastructure at the mines and the loss of jobs.
"All of this went on while Hulley, an advocate who is supposed to be a protector of the law and human rights, looked on," said Du Plessis.
On Wednesday, the presidency announced Hulley's appointment as Zuma's legal adviser on a part time basis. He has worked for the president in the past.
Hulley represented Zuma when he faced charges of bribery and corruption in relation to the controversial arms deal.
Solidarity called on the parliamentary portfolio committee on mineral resources to support its demand for the "immediate rescission" of Hulley's appointment.
The committee recommended last Friday that criminal charges be brought against all Aurora directors, the union said.