Break-and-enters have sharply increased in the Castle Hill region this year.
Castle Hill police crime manager Detective Inspector Gary Bailey said they had mainly occurred at Castle Towers and in Baulkham Hills and Beaumont Hills.
He said the break-ins had increased from last January.
"But there were 24 break-and-enters and four cars stolen since just last Thursday," Inspector Bailey said.
He said police believed about a dozen youths, all male, were largely responsible.
"We've had reports of young people, [aged] 14 to 18, in the vicinity of the break-and-enters," Inspector Bailey said.
"Two were described as Caucasian, one as Indian in appearance and one as Pacific Islander."
He said the thieves had mainly stolen food, alcohol and perfume.
He also said police would speak to the parents of the young people believed responsible.
"We're asking people to be aware of these crimes and to make sure their homes are secured, particularly when they are away," Inspector Bailey said.
"Report any suspicious activity you see, as well.
"If you see people repeatedly knocking on doors or lurking near premises, please call us."
Castle Hill command: 9680 5399. Crime Stoppers: 1800 333 000 or http://www1.police.nsw.gov.au/