Wearing matching knitted beanies - one in blue and the other pink - Sam Neill's "brand new" granddaughters have made their first public debut.

The Jurassic Park star introduced his identical twin granddaughters, Molly and Poppy, on Twitter this afternoon with a photo him grinning while he cuddled what some followers have called his "gumnut babies."

His tweet was soon flooded with hundreds of comments congratulating him and his family on their new arrivals.

English food writer and cooking show host Nigella Lawson was among the comments. She wrote: "How deeply wonderful".

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New Zealand actress and comedian Madeleine Sami also posted: "Congrats sam!!!".

At 8.50pm, the tweet had received 1.2K comments, 614 retweets and 24,399 likes.

Neill's big screen career spanning over more than 40 years includes lead roles in Hunt for the Wilderpeople, Peaky Blinders and Jurassic Park.

Born in Northern Ireland, the actor moved to Christchurch with his family in 1954.