"This is it, wish us luck! (My partner and I cycled because there wasn't enough room in the car for the support crew ... but it also put me in the best possible mood!)"
Genter included the hashtags: #42weekspregnant #cycling #bicyclesarethebest.
A third snap pictured Genter standing beside her partner, Peter Nunns, each grasping the handlebars of a bike with a bright pink helmet in her bike basket.
Genter later said the ride was mostly downhill.
Speaking at a Green Party conference in Palmerston North, party leader James Shaw called Genter's decision to cycle to hospital "very on brand".
"I'm very proud to be a member of a party that supports that, and proud to be living in a country where two members of the country's executive are able to have children as part of their job," Shaw said.
"I just think it's just fantastic news, and we wish her all the very best."
When asked whether he would let his wife ride a bike to the hospital, Shaw said it wasn't up to him to tell his wife what to do.
MP Eugenie Sage also weighed in on the announcement.
"Julie Anne has been exercising, so it's totally on brand," she said.
Genter's post, which was uploaded just before 11am, was tagged at Auckland City Hospital.
Dozens of people had commented with good luck messages, others commenting on the fact she had cycled and calling her a "rebel".
The news comes a little under two months after Prime Minister Jacinda Ardern gave birth to her first child, baby Neve.
Ardern had Neve in Auckland Hospital on Thursday June 21.
She has recently returned to her role as Prime Minister, following six weeks of maternity leave.
Genter has previously said she would stop flying and base herself in Auckland from July 15 following the birth of her child.
It will be Genter and Nunns' first baby.
Greens co-leader and Climate Change Minister James Shaw will take on Genter's transport and health portfolios while she's on leave, while Conservation Minister and fellow Green MP Eugenie Sage would become Acting Minister for Women.
