Joshua Alan James Wootton appeared before Sydney's Downing Centre District Court where he pleaded guilty to three charges.
These comprise recklessly damaging property, stalking with intent to cause physical or mental harm, and using a tracking device to see the victim's location without her consent.
According to agreed facts filed with the court, the 33-year-old destroyed the woman's mobile in September 2015.
She admitted a friend had "pecked" her on the lips after driving her home from a party.
Wootton confronted her after viewing the "Find My Friends" app and seeing that the vehicle she was in had pulled over on the roadside on the way home.
After the victim told him it was "nothing more" than a peck, he became angry and snatched the phone out of her hands.
"He squeezed the phone in his hand so hard that the screen broke and shattered," the agreed facts say.
Wootton also pleaded guilty to stalking the woman in February 2019 by appearing at a jewellery store where she worked with her partner.
"As (the victim and her partner) were driving out of the car park they saw the offender standing nearby and peering over a fence, looking toward the jewellery store."
The ex-officer also admitted installing a GPS tracking device on the woman's car from January to December 2019 without her knowledge.
Wootton, who resides in Oran Park in Sydney's southwest, became a serving police officer in December 2019 and was based in the city's eastern suburbs.
He was stood down from his role in June 2023 and will be sentenced at a later date.
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