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Mr Tong (62) made the announcement yesterday morning after informing Sir Tim of his intentions late last month.
At the last round of elections, held in October 2019, Mr Tong said it would be his final as mayor of Southland, after three terms in the job.
But last November he decided he would stand for the Invercargill mayoralty at the 2022 election.
It took him almost a year to make the news public, but he said he had good reasons for standing.
"I just felt there needed to be continuity," Mr Tong said.
His campaign would be centred on giving attention to surrounding areas he believed were neglected, including Bluff and Otatara.
Meanwhile, the news has been well received by the long-standing Invercargill mayor.
Sir Tim, who is in his ninth term, said it was good of Mr Tong to contact him last month and give a heads-up.
"I think it’s great. My motto is ‘the more the merrier’.
"It’s one of the advantages we both have, we’re a known name," he said.
"The only similar situation I’ve been in was the folk singer Suzanne Prentice [who ran for mayor in 2010], and a lot of people thought she would roll me ... but it didn’t work out."
However, he also said he enjoyed having more competitors because it led to a "lively contest".
"Full of surprises, elections," he said.
"You just don’t know what people are thinking."
Sir Tim’s ninth term as mayor of Invercargill has been marked by challenges.
In May, his driver’s licence was suspended. He later revealed he had pulled over on yellow lines after swerving through a roundabout.
In August, he claimed he had been left "traumatised" by workplace bullying during a tumultuous week at council where his emails were intercepted and used against him at a closed meeting.
Mr Tong said the issues facing Sir Tim in the past year were not on his radar.
"I’ve got the greatest respect for Sir Tim and I saw him personally and told him what I was up to."
Mr Tong lives in Riverton, 40km west of Invercargill, but is not required to move if successful next election.