As of Friday morning, 29,400 people had put their name to the online petition started by Sheena Ross, Jackie Coursey and Candice Milner from Rata Midwives.
The midwives say they have been inundated with messages from people wanting to help them keep the birthing unit open.
"We’ve had phone calls, emails and the number of signatures has been increasing steadily, especially after Gemma McCaw became involved," said Sheena.
"She has personal experience of being able to make a choice when it comes to where you have your baby, and Gemma knows understands how important that is."
The closure of St George's Maternity Centre would leave Christchurch Women's Hospital as the only birthing facility in the city.
Gemma had postnatal care after the birth of both her daughters, three-year-old Charlotte and 10-month-old Grace, at St George's.
Gemma and Richie McCaw have both signed the petition. Sheena said primary birthing services in the city should be increasing their capacity, not reducing.
"The Burwood primary birthing unit was the most well-used primary unit in the city," said Sheena who worked at Burwood until the primary unit closed.
"When it closed in 2016, St George’s was then the only one in the city and it is often over-subscribed.
"Giving birth at a primary unit or at home enhances outcomes for well-women with uncomplicated pregnancies.
"We are at a loss to understand why another key part of women’s health services is under threat, yet again.
"Not only are the outcomes better for mums and babies, but the whole whānau is a key part of the experience.
"Primary units are not like the main hospital; they provide a home away from home environment with the expert support of midwives.
"The loss of post-natal care option in a central city primary unit with St George’s current reduced service is reducing options for women.
"We should be building more primary birthing service capacity; in fact we should have done this before Burwood closed six years ago.
St George's will reveal its final decision at the end of April.
