The Instagram announcement shows the couple with their two young sons at St Heliers.
Well wishing quickly ensued on the post with The Hits host Toni Street commenting on the "awesome news".
"Welcome to the party of 5 club, it's wonderful," she said.
Street made a quick hospital dash just days ago to welcome her new baby boy, who was carried by surrogate Sophie Braggins.
Her new bub, Lachlan Stephen France, is the third child for Street and husband Matt France.
Family is the most important thing in the world to me. With @honorcarter being the best and most dedicated wife/mother, it excites me to know we are adding to our family. #BabyCarter #BabyNo3 #Partyof5
Posted by Dan Carter on Saturday, 11 August 2018
Netballer Kayla Whitelock also commented on the post, congratulating the couple on the "awesome news".
Carter is about to begin the next chapter of his celebrated rugby career with a move to Japan.
He has signed a two-year deal with the Kobe Steelers.
The former All Blacks playmaker made his test debut for the national side back in 2003.
Carter played for the same provincial and Super Rugby teams as former All Black skipper Richie McCaw who similarly announced on Instagram last month that his family is growing by one.
It will be the first baby for McCaw and his wife Gemma.
