According to The Daily Telegraph, the expectant mum is around three months along.
"They have told their close friends they are expecting again. I’ve heard she fell pregnant eight weeks after giving birth to her daughter in August," a friend revealed.
Joanna and George currently have two children, Boston, 22 months, and Birdie, six months,.
The announcement comes just days after George's twin Tom, and his partner, Tahlia Giumelli, shared the news they were expecting.
"I'm three months along, so we've just hit that 13 week mark," Tahlia said.
"I'm showing a little bit, but I don't know if it's just all the food I've been gobbling up or if it's a Burgess baby, they are quite large kids."