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Invercargill city councillors have described her departure as a significant loss for the city.
However, some of then looking to the future. One has already put her hand up for the job and another intends to challenge Sir Tim Shadbolt for the mayoralty in the next election.
Ms Biddle’s resignation will take effect later this month. She said her decision was not taken lightly and it had taken some time to come to terms with.
"My time at council has been both incredibly fulfilling and, at times, challenging. In the past 12 months, I have experienced difficulties within our council and in my personal life, which has led to my decision to leave."
Her resignation comes amid a probe into the council by the Department of Internal Affairs, which has raised concerns about "significant conflict".
Ms Biddle was also taken to hospital in August, for an unknown reason, and was the subject of a code of conduct complaint in the same month.
In recent times, questions have also been raised about Sir Tim's performance.
Otago Daily Times reporters were not able to reach Sir Tim for comment last night, but Ms Biddle acknowledged him in her departure statement.
"His Worship has been a sincere gentleman who has demonstrated to me on many occasions his genuine love for people and I wish him and the current elected members all the very best for the future."
Fellow councillor Nobby Clark said Ms Biddle’s departure would be a big loss for Invercargill.
"She is a very compassionate person ... [an] extremely hard worker who’s done an absolute heap of work supporting the Mayor."
Cr Darren Ludlow said it was always sad to see somebody leaving the council.
"It has been a trying time, and the role of deputy mayor is more demanding than many people realise."
Crs Lindsay Abbott and Graham Lewis said it was sad to see her go, and Cr Peter Kett said the news was a major surprise.
"I’m actually shocked. I just didn’t believe my eyes.
"Toni stood up for what she believed and she didn’t flinch. It is a sad, sad loss for the city."
Cr Alex Crackett said she became very close to Ms Biddle during their time at the council.
"It is sad, the environment we are in. She has been caught up in a situation with a lack of leadership at the top.
"She had to overcompensate, which resulted in a lot more stress to her in the role than if she had been supported by a fully present mayor."
Ms Biddle’s resignation means a new deputy mayor will have to be selected, and will result in a by-election.
Even recognising it was stressful job, Cr Crackett admitted she would go for it.
"I think I have got something different to add in the situation ... and I would work extremely hard."
Crs Kett and Lewis did not rule out an interest in the position, but Crs Ludlow and Clark agreed it was the Mayor who would choose.
Cr Ian Pottinger went further and said he planned to stand for mayor in the next election.
"There’s no point to mucking around with deputy mayor."
Crs Lindsay Abbott and Nigel Skelt said they were not interested at all in the job.
Crs Rebecca Amundsen, Allan Arnold and Lesley Soper could not be reached for comment.
Chief executive Clare Hadley said the council would consider the position of deputy mayor at the October 27 meeting.
The by-election process must also begin and Mrs Hadley said it was likely to run into February.
A preliminary timetable would involve nominations opening in November and closing in December, with voting open between late January and mid-February.