Garner will continue to present the RadioLive drive show, from 3pm to 6pm weekdays. The new show will be simulcast on RadioLive.
Garner said he expected fun, laughter and entertainment.
"I see this as a great chance to be part of a programme that will reflect our country and our lives - no matter where we live.
"And when those in power in Wellington, or in your own community, make bad decisions, mislead you or are hypocrites, I promise - you can rely on us to keep them honest - simple."
Du Plessis-Allan, a Herald on Sunday columnist, said she was excited to be part of the show.
"Duncan is one of the best journalists around so I'm stoked to be able to learn from him. But Duncan, if you're reading this, don't let it go to your head. I'm not your sidekick. In fact, they told me you're my sidekick."
MediaWorks chief executive Mark Weldon said du Plessis-Allan and Garner were two of the most engaging and talented journalists in the country.
"Together they represent a major investment by MediaWorks in the future of current affairs leadership in New Zealand."
On social media yesterday, some were excited for the new duo but others were less sure.
Read more: Kiwis react to Campbell Live's replacement
Although at least one person said they had been "converted" back to TV3, others questioned why Campbell Live was axed only to be replaced by another current affairs show.
Du Plessis Allan has some experience with the 7pm slot - she was once a correspondent on rival current affairs show Seven Sharp. She was accidentally caught in discussion with MediaWorks boss Mark Jennings before officially announcing her switch from TVNZ.