Libby said she was over the moon to be cast and she identifies with the story and her part in it.
"What I love about Poppy is that she's ambitious and wants a career. I do too," she said.
"Poppy doesn't care about what people think of her and the fact she has Down syndrome. I really relate to Poppy. We could be sisters."
As the story of Poppy includes learning to drive and working in a garage, Libby will be preparing for her role over summer.
The preparation, as well as drama coaching will be supported by a grant from the IHC Foundation.
The New Zealand Film Commission is supporting Poppy with funding from the 125 Fund created to celebrate the anniversary of women's suffrage.
New Zealand on Air has also offered funding alongside private supporters and sponsors and TVNZ will screen the film in New Zealand.