Australia's second-longest serving federal politician is running for Mayor of Hornsby.

Philip Ruddock is coming out of retirement amid “negativity” within the NSW Liberal Party over botched council mergers.

Mr Ruddock was first elected to Parliament in 1973 and retired last year.

He now says he is entering the fray in the Hornsby Shire elections in September as the Liberal candidate for Mayor.

“Some people think when you reach a certain age, you retire,” Mr Ruddock, 74, said.

“I've never seen it that way and I will always look for a role as long as I'm fit and able.

“I think we have an ageist problem in this society and if I can demonstrate you can contribute positively even in your mature years then I'm probably doing a good thing.”

The move comes amid in-fighting within the Liberal Party over the NSW Government's backdown for the proposed merger of the Hornsby and Ku-ring-gai councils.

Current Hornsby Mayor Steve Russell has criticised the handling of the mergers, saying his town faces a $200 million revenue shortfall over the next decade as a result of the change.

Mr Russell is stepping aside and make way for Mr Ruddock to run.

“The Liberal Party's told me that I'm toxic so they don't want me anywhere near it [the campaign] so they told me to go fishing,” he said.

“It's a bit sad to be stepping down under these circumstances.

“They urged me not to run and told me that my brand was toxic because of my stance on amalgamation.

“I would've liked the opportunity to go to the people and to see what they thought of the job I've done in the last four years.”