Breakfast weatherman Matty McLean posted a photo of the two presenters together with the caption: 'WE'RE HAVING A BABY!'
He clarified: "Well, Hayley is. But given how clucky I am, I'm tagging along for the ride whether she wants me or not.
"Congratulations @theholtbolt - you're going to be the most incredible Mum x (sic)."
Melissa Stokes wrote on Instagram: "Hayley!!!! I'm so thrilled for you...as excited as when I found out I was having a baby,".
Political reporter Jessica Mutch McKay, who welcomed her first child with bodyguard husband Iain McKay also congratulated Holt.
"Yay! Congratulations - such exciting news. Now for the wild ride of being pregnant on TV!"
Mutch McKay continued her work on-air until she was 38 weeks pregnant.
European Correspondent and former Breakfast newsreader Daniel Faitaua wrote: "Your best scoop my friend! Congrats."
On Facebook, viewers also expressed their excitement.
"Huge congratulations Hayley, you will be such a gorgeous mum. So loving and caring. It's an exciting year for you already," a viewer wrote.
The viewer joked fellow Breakfast host Matty McLean might be asked to babysit.
Others were quick to ask who the father was - something Holt did not reveal today.
One person wrote on Facebook: "Question is, who's the Dad lol."
Another commented: "Who's the Daddy??? Only asking because I know you've had unhappy relationships in the past, and I always hoped you'd meet someone special and right for you, because you're an amazing person [sic]."
Others were less impressed with the news:
"Wow Hayley is the only woman who has ever become pregnant! It must have been the immaculate conception!
"So what? About exciting as Jacinda's [Ardern]...and EVERYONE else's... (sic)."
