Moves by Victoria's human rights commissioner and the federal government to intervene in a controversial firefighters' pay deal is an "unprecedented hijacking", the union says.
Victorian Equal Opportunity and Human Rights Commissioner Kristen Hilton and Workplace Minister Craig Laundy have applied to intervene in the Fair Work Commission's decision on whether the Metropolitan Fire Brigade deal should be approved.
Ms Hilton is concerned the deal discriminates against women, parents, those with a disability and carers.
At a hearing at the Fair Work Commission in Melbourne on Thursday, lawyers for Mr Laundy listed objections to more than 40 clauses in the 321-page document.
Legal counsel for both the MFB and United Firefighters' Union argued there was no place for VEOHRC or commonwealth involvement.
"This is a total hijacking of procedures and I submit to you, an unprecedented hijacking," Richard Kenzie QC, appearing for the UFU, told the hearing.
He accused Mr Laundy of trying to "turn this proceeding into something like a royal commission."
Counsel for Mr Laundy, Jenny Firkin, said the minister had to maintain the integrity of the Fair Work Act and there were concerns for non-union members if the agreement was allowed.
The MFB and UFU are scheduled to sit before the Fair Work Commission in August over whether the controversial deal should be ratified.
Deputy President Val Gostencnik said on Thursday he would reserve his decision on whether the commonwealth and VEOHRC would join that hearing.
A decision on the intervention application is expected on Monday.