Arendse was drafted onto Sundowns' teamsheet beyond the prescribed deadline in their 1-1 Absa Premiership draw with Bidvest Wits over a month ago, and was subsequently fielded in the starting line-up.
Upon review of the matter, the PSL DC have since found the club and the player guilty of contravening the rules, but have reserved their sanction until defence submissions have been made by The Brazilians.
"The Premier Soccer League Disciplinary Committee has delivered verdict on matters involving Mamelodi Sundowns, Mr Wayne Arendse and Mr Pitso Mosimane," read a statement from The League.
"In the first matter involving Mamelodi Sundowns and Wayne Arendse, the Disciplinary Committee found both the club and the player guilty of misconduct.
"This is after Mamelodi Sundowns fielded Mr. Arendse whilst he was ineligible to play in the Absa Premiership fixture against Bidvest Wits on 07 October 2018.
"Sanction will be handed down once all the parties have made submissions to the Disciplinary Committee."
Meanwhile, coach Pitso Mosimane has also been found guilty over the incident in which he was accused of assaulting one of AmaZulu's officials, with the DC similarly reserving the sanction.
"In the second matter arising from an incident between AmaZulu and Mamelodi Sundowns on 01 September 2018, Mr Pitso Mosimane was charged with misconduct after he allegedly assaulted AmaZuluteam security official, Mr. Satchmo Ngwenya by punching him in the face," further read the statement.
"Mamelodi Sundowns was charged for bringing the League into disrepute as a result of their coach’s actions.
"Both the club as well as Mr Mosimane were found guilty. Sanction will be handed down once all the parties have made submissions to the Disciplinary Committee."