Bennett posted to her Facebook page to declare the news.
"Hi. As per the Herald article, I have gone public with my decision to have weightloss surgery. I had a gastric by pass and am at the beginning of what I plan to be a much healthier active life. After years of weight gain and loss I can see a clear path ahead," the post read.
"I feel great, paid for it myself and thought long and hard about whether to go public. On one hand it's no ones business but mine but as a woman who has battled with my weight all my adult life I didn't want the innuendo and I wanted to be upfront with others who struggle about the choice I made."
"I had hoped for an inch about it in the gossip page (as it is gossip not news!) but they have run it bigger - I guess that's public life! The best reason for going public is I feel great about my decision and don't care what others think."
"So haters can hate - I'll just keep ignoring them and do the best I can both personally and professionally."
A gastric bypass surgery consists of majority of the stomach and a small portion of the intestine being disconnected from the passage of food.
Bennett is not the only politician to undergo this surgery, former Prime Minister's Dame Jenny Shipley and David Lange have also undergone types of gastric surgery.
Ex MP Dame Tariana Turia, also got her stomach stapled back in 2010.
According to Stuff, Bennett is recovering well.