Canberra: Sexual misconduct allegations against the City of Melbourne’s longest-serving lord mayor, Robert Doyle, are “very serious claims”, the Victorian premier, Daniel Andrews , has said.

Doyle issued a statement on Sunday announcing he would take leave for a month as an independent investigation was launched into allegations he engaged in sexual harassment and inappropriate conduct. Doyle described the allegations as “thoroughly abhorrent”.

“My decision to take leave must not be interpreted as any concession or admission,” the statement said. “What is important now is that proper process be allowed to take place and that the rights of all involved are respectfully handled.”

On Monday, Andrews said he hoped the investigation would be quick.

“It’s very important for everybody involved, given the seriousness of these issues, for that process to be a quick one and a thorough process,” he said.

The allegations were made by councillor Tessa Sullivan, a lawyer who ran on the same ticket as Doyle in the 2016 election. She resigned from the council on Friday after making a complaint that Doyle had grabbed her breast and had made explicit comments about sex to her.

A statement issued by the council said Sullivan had resigned “after herself and other women had experienced repeated sexual harassment which had made their workplace intolerable”.

Jackie Watts, a long-serving City of Melbourne councillor who is currently on sick leave, expressed her support for Sullivan, telling radio 3AW on Monday she and other staff were “all shocked and of course Tessa is feeling it greatly”.

“It takes a very brave woman to speak up in this way and of course it puts everybody in a very difficult position,” Watts said.

“Against a lord mayor with this experience and with a well-educated and capable and sophisticated councillor, one has to take these allegations very seriously. I am confident that the CEO at the City of Melbourne is taking this very seriously indeed.”

The City of Melbourne chief executive, Ben Rimmer, said an independent external investigation would be conducted by Dr Ian Freckelton QC.

“Pending the outcome of this investigation, I will determine what further action, if any, is required and available to me,” Rimmer said.

“The investigation will be conducted in accordance with natural justice principles. These allegations are very serious and, if proven, would be inconsistent with the values of the city and the organisation. Therefore, it is very important that these matters are thoroughly investigated.

“We will not be commenting further on this matter at this time.”