The Health Workers Union says Ms Mikakos has failed to consult with the sector and lacks a basic understanding of her portfolio.

"Sadly, our union's relationship with your government is now officially dead, due solely to the breathtaking incompetence of your current health minister," the union, aligned with Labor's right faction, said in a letter.

Its secretary, Diana Asmar, called on Ms Mikakos to resign.

Premier Daniel Andrews rejected suggestions his health minister was incompetent and didn't understand her portfolio.

"All ministers are fully aware of the accountability for those matters that go on within their portfolio," Mr Andrews told reporters on Thursday.

The Australian Nursing and Midwifery Federation also came to Ms Mikakos' defence.

The union, which represents about 92,000 members, said the attack was simply factional warfare within the Labor party.

It came as Ms Mikakos fronted a judicial inquiry into Victoria's hotel quarantine program, which sparked the state's second wave of COVID-19.

She distanced herself from the decision to use private security guards to watch over returned overseas travellers.

Australian Associated Press