Two children, aged three and five, were in a Sydney unit when their mother died in suspicious circumstances, say police.

One of the children called police to the home on Moorefield Avenue, Hunters Hill at 11.30am today.

Officers found the body of their 30-year-old mother in the unit, and the two children.

Detective Inspector Richard Puffett of Gladesville said police were treating the death as suspicious, but he would not comment on how the woman died or whether the unit had been broken into.

A neighbour and friend said the victim lived alone with her two daughters.

She was a Christian and regular churchgoer, a dedicated mother, and a keen runner who competed in the City To Surf and was training for a marathon.

"She was always by herself with the kids," she said. "Sometimes I look after the kids. She's nice, she's very quiet. It's really a shock now."

Anyone with information should phone Crime Stoppers on 1800 333 000.