"Two incredible years full of tragedy and triumphs - especially when you add the Royal Wedding, the Rugby World Cup and the Olympics into the mix.
"It's been a rollercoaster ride.
"Breakfast is bloody hard work and it takes a lot of effort from an incredibly committed group of people to make it happen.
"I want to say thanks for having me on the team. Bring on summer and 2013."
TVNZ head of news and current affairs Ross Dagan said Bagust was "keen to sleep in past 3.50am and spend more time with her young family".
Dagan told staff this morning the search to fill Bagust's empty seat would "get underway shortly".
"I have high regard for Petra's broadcasting skills and she has my warmest thanks for her contribution to Breakfast over the last two years," Dagan said in a statement.
"I respect her decision to put family first ... and return to more normal hours. We all wish Petra well."
Bagust will remain on Breakfast until the programme finishes for the year on December 21.