Mgidlana this past weekend released a statement in which he slammed the Nehawu of character assassination, saying the union's claims that he erroneously awarded himself a bursary and a bonus of R70,000 were "misleading and disingenuous".
Nehawu, which is currently engaged in a dispute with Mgidlana and senior management over the 2017 salary increases, on Tuesday hit back.
The union has given Parliament's presiding officers, including Speaker Baleka Mbete, until the end of the week to suspend Mgidlana pending the outcome of an independent investigation.
The union said it would provide documentary proof of Mgidlana's transgressions.
"We want to assure him that we have irrefutable hard-core evidence to prove our case and kick him out of Parliament because he doesn't deserve to be here. He lost his legitimacy the first day he set foot here," said Sthembiso Tembe, chairperson of the Nehawu's parliamentary launch.
While tabling last week's parliamentary budget, Mbete said a parliamentary audit would conduct a probe into the allegations against Mgidlana.
The union said it was against a probe by internal audit staff as they reported to Mgidlana.
The union wants an independent investigation, adding Mgidlana should not be allowed on parliamentary premises during the probe.
"This suspension must be coupled with its usual conditions. Failure to do that lends credence to the view that Mgidlana is untouchable and is accorded preferential treatment," said Tembe.
Mgidlana on Sunday denied that staff would not get their annual salary increases, appealing to staff to "return to the negotiating table".
The unions said its previous meetings with management had been reduced to "tea sessions" with no offers being put on the table.
"Going forward they have to call us when they have a new tangible position in accordance with the directives of the presiding officers." Tembe said
Mgidlana's claims that Parliament was awarded a smaller than expected budget with a shortfall of over R1 billion was not an excuse to deny workers a pay rise.
"Parliament was warned to exercise austerity measure about three years ago. Instead of cushioning the limited resources of Parliament as advised by the National Treasury, Mgidlana expanded his support staff with gay abandon and created positions that were not part of the structure of Parliament."
Nehawu said it was still awaiting feedback from the public protector after the union laid a complaint in July last year.
The union said it intended taking Mgidlana to the cleaners. If he is not removed and Nehawu does not get any joy from the Public Protector, the union vowed to approach the courts for an order to declare Mgidlana unfit to hold office.
Nehawu regional secretary Baxolise Mali weighed in, asking presiding officers to stop treating Mgidlana like "a golden boy".
Labour relations at Parliament has been at a low for the past few years. In 2015, workers went on strike over, among others, bonuses and working conditions.