The DA says Patricia de Lille "must come clean" about an SMS she allegedly sent to one of her councillors during the process of appointing a new City manager in September 2016.
News24 first reported about the alleged SMS in January. It was used as one of the key pieces of evidence against De Lille in the party's internal "Steenhuisen report".
De Lille previously requested that the SMS be submitted to the court during a review of the report. She has been waiting for it since last year.
Last week, the Mail and Guardian reported on the issue again with a screenshot of the SMS, allegedly sent by De Lille to mayoral committee member Xanthea Limberg during the process to elect Achmat Ebrahim as City manager.
It allegedly reads: "I want to keep Achmat so score him highest."
"If De Lille strongly believes that the SMS that she sent Limberg, clearly showing her interference in the appointment processes of Achmat Ebrahim as Cape Town City manager, does not hold up, she should submit her cellphone for forensic analysis," DA spokesperson Natasha Mazzone said on Monday.
"If she did not send the message, then she should have no problem confirming this by allowing the analysis."
Mazzone also slammed De Lille for comments she made in the Cape Times on Monday, suggesting Limberg and fellow councillor JP Smith plotted together to bring down de Lille "under the pillowcase".
Smith and Limberg are known to be dating.
"This kind of behaviour is deplorable. It is inconsistent with a DA government and a DA mayor," Mazzone continued.
"The party holds all its members of the executive to the highest standard. As such, this text message alone should have been enough for De Lille to resign as she clearly was not fit to be a mayor.
"To then slander Limberg and bring her personal life into what is essentially a criminal matter, is the worst kind of sexism and racism."
Mazzone has previously said that the party did not release the evidence to protect the whistleblower, who at that point was only known to the party and afraid of victimisation.
Mazzone said the party was of the view that the SMS was real and De Lille must "come clean" about it. She also rebuked De Lille for not categorically denying she sent it.
De Lille told News24 on Monday that the onus was on the party to submit the SMS as evidence before her disciplinary hearing, and in her court challenge, as she has been "requesting it for months".
She said she and her lawyers had seen the SMS for the first time on Friday.
"I've said several times, the onus was on the DA to prove this [SMS] in a disciplinary committee set up by themselves.
"In the hearing and before it, we asked them for the name of the person, the number it was sent to and a copy of the SMS. It was never forthcoming as it needed to be tested.
"All of a sudden the SMS just appears. I'm not going to go into whether ... she must answer, not me. I am surprised it is coming out now. That is the question I want answered.
"It could have been tested already had it been provided a long time ago."
When pressed on whether she denies sending the SMS, De Lille said she did not want to discuss the merits of the issue, as she wants to answer for it in her disciplinary hearing.
As for her comments on Limberg and Smith, she did not have a detailed response, and said the pair must answer for themselves on the allegations and "smear campaign" they've made against her.
Smith told News24 on Monday that it was not new news that he and Limberg were close. The matter at hand was the SMS and the evidence against De Lille.
"It is an outrageous statement," Smith said of De Lille's accusations that the pair were plotting against her.
"Somewhere between 30 and 50 councillors testified before the Steenhuisen committee. So there is no way that I could have steered this myself.
"The anger and resentment about the mayor is widespread throughout the caucus, evidenced by the fact that 70% of councillors voted for the motion of no confidence in her."
Smith said De Lille was very skilled at "media strategies" to distract from the issues and was going on ad hominem attacks on individuals.
His relationship status, for instance, was completely irrelevant to the material allegations against De Lille.
News24 could not reach Limberg for comment on Monday.
She has previously maintained that the SMS was authentic and "inappropriate" and had reported it to the party through various processes, the Mail and Guardian reported.
The Western Cape High Court meanwhile will deliver its judgment in the urgent court tussle between De Lille and the DA, regarding the cessation of her membership, on Tuesday.