He lodged his application just minutes before the deadline closed on Tuesday.
It comes after former deputy mayor Toni Biddle announced her resignation from the council in October.
Lush said he saw the opportunity and thought it would be a good one.
"I've put a huge amount of thought into this decision. I am excited about the campaign and learning curve ahead if I am elected."
The 55-year-old has lived in the South for 18 years and said his passion lies in developing youth spaces and affordable housing.
"I look at things like the Margaret Mahy playground in Christchurch and I think it's time we updated some of our playgrounds here."
Lush said he would continue with his nationwide show on Newstalk ZB Marcus Lush Nights if elected.
"I've made this decision with the approval of my bosses. I am certainly not running for mayoralty because I already have a fulltime job.
"However during the campaign, I will obviously need to be impartial so will avoid the topic altogether."
Confirmed candidates in the running for the seat are: Wade Devine, Tom Downey, Simon Edwards, Bernadine Goldsmith,Wayne Harpur, Carl Heenan Kevin Mulrooney and David Pottinger.
Deputy Electoral Officer Michael Morris said he was delighted with the high interest in the position following Ms Biddle's resignation.
Election signs could appear from December 23 and voting will open on January 26 in 2021, with election day on February 17.
- additional reporting by Kerre Waterworth
