Nancen Beryl Chauncy (1900-1970) was born, one of twins, in Northwood, Middlesex, England.  When she was 12, the family moved to Hobart in Tasmania after her father, a civil engineer, lost money and needed to find employment.  He worked as a council engineer, and Nan went to the Collegiate School in Hobart.  By 1914 the family had moved to Bagdad, north of Hobart, and through sheer slog cleared land to grow  apples.  According to the Australian Dictionary of Biography:

“Nan, who left school at 16, was to write of her childhood as a golden age: she enjoyed the close, family teamwork, the stories told by lamplight, the discovery of fauna and flora, and the legend of a bushranger's cave nearby.”

She was a keen girl guider, and when her job with Cadbury’s sent her to England in 1920, she used the time to learn more about guiding.  She spent the next few years travelling Europe and teaching.  Sailing back to Australia in 1938, she met a German refugee, Helmut Anton Rosenfeld, whom she married.  When the second World War started, they adopted the Chauncy surname because of anti-German feeling.  To make money, she wrote radio scripts and articles, and had her first novel, They Found a Cave, published in 1948.  She won the Children’s Book of the Year Award three times, and was notable for her realistic stories with an emphasis on conservation - she and her husband bought 1000 acres of bush and made it a wildlife sanctuary.

The Children’s Book Council of Australia presents the Nan Chauncy Award for an outstanding contribution to children’s literature in Australia.

As far as I know, only one of her books is pony-orientated:  The Skewbald Pony is aimed at younger readers, and is the story of Carrie, who lives on a sheep farm.  The book is very hard to find.

Sources
Australian Dictionary of Biography - entry on Nan Chauncy
Wikipedia entry on Nan Chauncy
Heather Chauncy (Nan’s daughter) on her mother

Nan Chauncy

The Skewbald Pony
Nelson, London, 1965, illus David Parry

Aimed at the younger reader, this is about Carrie, who lives on a sheep farm.
 

Bibliography - pony books only