Cr Biddle is in her first term on the council. She was the third-highest polling Invercargill city councillor in the 2016 local body elections.
The 42-year-old was just over 100 votes behind the highest polling councillor, Darren Ludlow.
Mr Shadbolt had already confirmed he would stand again at the October election and now has an opponent, Cr Biddle.
At the last election, Mr Shadbolt had two people stand against him in the mayoralty race, Karen Arnold and Tom Conroy.
With 11,618 votes, he gained close to 5000 more than Mr Conroy, who was the next highest in the polls.
Cr Biddle is the Southland Museum and Art Gallery trust board chairwoman.
The trust board this year decided to close the museum because of safety concerns over the Invercargill building.
