He has hosted the show since its inception in 2017.
He said he had decided to leave the "best job ever" because he was struggling with the "brutal hours".
It is understood Garner regularly starts works at 4am, well before the show screens from 6am on weekdays.
"I am so proud of what we have achieved in all the teams I've been a part of and in this tough, hostile business you're deluded if you think you do it alone," Garner said in a statement.
"Three and now Discovery has the most talented staff and best journos I've ever worked with. Their commitment, knowledge, sources, sheer guts and work ethic is legendary and continues to be so."
Garner said he would miss working with co-hosts Mark Richardson and Amanda Gillies.
"This place has been home for 20 years and it's bloody hard to say goodbye but I have to. I have a few important family and personal things that need my attention and now is the right time to go."
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Comments
Well... This will lift the quality of NZ news and current affairs reporting significantly. Something that is sorely needed especially in the Newshub group.
Although I doubt they'll ever challenge ODT's position as a news agency no matter who they replace him with.
I'm pleased to see him go.
I hope Amanda will soon follow Duncan's lead! Her constant overemphasizing of insignificant words, and smug celebration of any action completed by a female, drives me batty.
Yes, I know I can switch channels, but you only have to watch TVNZ for a few minutes to feel the onset of violent diarrhoea.
What a good news story. He is so biased and opinionated. Just hope he's not headed for something in prime time viewing with Mediaworks.