Melbourne Lord Mayor Robert Doyle is under investigation for declaring a $40,000 donation he made to his own election campaign more than a year after the poll was held.

In yet another blow for the embattled mayor, Victoria's local government watchdog is probing the belated disclosure, which was made on January 8 while Cr Doyle was on leave amid an inquiry into sexual harassment allegations by a former female councillor.

Melbourne Lord Mayor Robert Doyle Credit:Paul Jeffers

"I write to update the 2016 election campaign Donation Return received by Team Doyle as I contributed an additional $40,000 to the campaign," Cr Doyle wrote in the declaration this week.

The declaration has prompted an investigation by Victoria's Local Government Investigations and Compliance Inspectorate.