Birthday time. Libby Hicks, centre, with daughters Sylvie and Isla who, respectively, turn two on September 30 and four on September 29. The sisters will have a joint birthday celebration.

On Saturday, 2-year-old Sylvie Hicks from Petone and 14,200 other New Zealanders born on September 30 will celebrate the country's most common birthday.

It's an even more special day in the Hicks' household too, with Sylvie's celebrations being shared with her sister Isla who turned 2 on Friday.

Mother Libby Hicks said her daughters would be having a a joint birthday, a combined event today for family and friends where the sisters would together enjoy a shared main present.

SIMON O'CONNOR/STUFF September birthdays are the go.

"It's a big day for us."

READ MORE:

* September a busy month for birthdays in New Zealand

* New Zealanders set to celebrate mass birthdays as 10 most common days come up

The country will on Sunday end a 10-day super-peak birthday season celebrated by 144,000 New Zealanders, Statistics NZ said.

Learning their daughters were born on the first and sixth most popular birthdays was a surprise for Libby and Andrew Hicks.

"I had no idea and I don't really know of other birthdays around that time, even at the girls' childcare and kindergarten," Libby Hicks said.

"But when I gave birth to Sylvie the maternity ward at Hutt Hospital was really full."

A five-week period to October 19 also includes the 30 most popular birthdays, Stats NZ senior demographer Kim Dunstan said.

At the other end of the scale, analysis of birth data from 1980 to 2015 also shows that, with the exception of the leap year February 29, Christmas Day is the least common birthday, ahead of the following Boxing Day.

This is consistent with birth numbers being down at the time of all set public holidays, including Waitangi Day on February 6 (ranked 360) and Anzac Day, on 25 April (361).

The Stats NZ figures show 6219 babies were born on September 30 over the 1980-2015 period, 45 per cent more than the Christmas Day's 4281.

A 40-week countback from the second half of September takes New Zealanders to the Christmas-New Year holiday period.

It brings the obvious, if unproven, dynamic of the demographic consequences of those relaxed, happy times for New Zealand parents.

"If you work back nine months, yes there's an indication there," Dunstan said. "These are the sort of things that show statistics can be interesting."

New Zealand's consistent peak birthday activity varied to that in Australia.

"They have a smaller peak than us and overall the September-October peak isn't quite as pronounced there," he said.

* How common is your birthday day and how does it rank? See www.stats.govt.nz/birthday