Prince Louis celebrated in Parliament with a waiata

Prime Minister Jacinda Ardern led congratulations to the newest royal baby in Parliament today. Photo / Marty Melville

Parliament's first act after a two-week break was to pay homage to a baby.

Prime Minister Jacinda Ardern passed a motion this afternoon to congratulate Prince William and Catherine, Duchess of Cambridge on the birth of their third child, Prince Louis.

"Along with many people around the globe, New Zealanders have witnessed and celebrated the joy that the Duke and Duchess of Cambridge have clearly found with their children Prince George and Princess Charlotte," Ardern said.

"I'm very pleased to lead the House's congratulations on the birth of a third child to this close-knit family.

"I'm sure that the duke and duchess are cherishing this special time with their new son, and no doubt Prince George and Princess Charlotte are enjoying getting to know their new little brother."

Ardern spoke about New Zealand's gift to the royal couple – a baby blanket and a selection of New Zealand books.

Leader of the Opposition Simon Bridges said it brought him "heartfelt joy" to congratulate Prince William and Catherine on the birth of their "beautiful bonny boy".

"Can I say to Catherine: it is quite an extraordinary feat, really, to appear publicly only hours after the birth looking so radiant.

Britain's Prince William and Kate, Duchess of Cambridge smile as they hold Prince Louis while leaving the Lindo wing of London's St Mary's Hospital on Monday, April 23. Photo / AP

"And I'm sure that she, her husband, and the older siblings, George and Charlotte, have real, as I say, joy at the birth of their little one."

Green co-leader Marama Davidson sung a waiata for the couple, which she said she wrote for her own children.

She also expressed sympathy to Catherine for having to bear intense public scrutiny so soon after giving birth.

"Whatever circumstances you are born into or give birth under, it will never ever eradicate the complete turmoil and exhaustion that birthing a baby is for any mother, so I hope that Catherine finds some downtime to simply just be whatever shape she needs to be in to be able to care for a new baby."

Louis is fifth in line to the throne.

A close-up view of Prince Louis in his mother's arms. Photo / AP