Security guards employed to monitor returned overseas travellers inside Melbourne's Covid-19 quarantine hotels were having sex with guests, leading to Victoria's new outbreak, it is claimed.

On Wednesday, there were 73 new cases of Covid-19 in Victoria, 20 of which had no identified source, the highest daily number of community transmission since the beginning of the crisis.

Overnight, 36 suburbs across 10 towns were placed into lockdown to stop the spread.

Premier Daniel Andrews admitted there were a "handful" of breaches among staff at the hotels, which he says may have led to the outbreaks.

Claims security staff were sexually active with some of the guests, including those in isolation in the hotels, would form part of the investigation, according to the Herald Sun.

The allegations had been heard among police and government circles, as well as within the hotel industry, the Herald Sun reported.

Andrews said DNA tests showed a number of cases could be linked to "staff members in hotel quarantine breaching well-known and well-understood infection control protocols".

"That left me in no doubt that if not right now, but certainly back weeks and weeks ago, there was a significant ­infection control ­problem," Andrews said.

"That is unacceptable to me. I'm sure that will be unacceptable certainly to all of those who will be impacted by the restrictions that we have had to reimpose."

There are also concerns that some staff regularly worked shifts at different hotels, as well as private work outside of the quarantine hotel contract.

Victoria's deputy chief health officer Annaliese van Diemen previously admitted there had been breaches of physical-distancing measures at the Stamford Plaza hotel.

"There's been some closer mingling than we would have liked of these guards in the workplace," she said.

"There's a large cohort of security guards and workers and unfortunately it does appear that quite a few of them have worked for single or multiple days whilst infectious."

It is also alleged security firms engaged in "ghosting" - a practice that involves inflating the amount of guards listed on duty in a bid to charge the government more for their services.

The Herald Sun claims workers with fake names were put on the payroll, leaving hotels dangerously understaffed.

Allegations to be investigated include:

• Hotel staff sleeping with guests.
• Guards shaking hands with each other and sharing lifts.
• Quarantined families visiting other guests within the hotel to play card games with each other.
• Limited training being offered to security guards prior to shifts.
• Security wearing personal protective equipment for extended periods of time without changing it.