Teenager Hayley Wormleaton was relaxing in her bedroom on a Tuesday night in 2015 when her grandmother called her name.
Ms Wormleaton came into the loungeroom to see her uncle holding a knife.
She and her cousin fled next door but watched as her uncle murdered their grandmother and youngest cousin on the neighbour's front lawn.
Their murders followed years of violence in the household, she said.
Now Ms Wormleaton is working with two NSW police sergeants to launch an app helping other Australians escape the perpetrators of domestic violence.
The "Empower You" app includes a private diary, a coded emergency signal feature with a GPS tracker, easy access to Triple Zero for emergencies and links NSW support services.
The app requires a passcode for entry and features a simple-looking icon to avoid sparking an abuser's suspicion.
NSW Police Commissioner Karen Webb said the app allows "victims to easily document abuse, which is often difficult to recall when in distress or when it occurs over long periods of time".
The app's diary feature could be used to compile photos of injuries and property damage, screenshots of emails or messages and times, dates and locations of abuse.
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"This is especially important in terms of coercive control, where we know a victim often feels helpless, so with 'Empower You', the abuse is clearly demonstrated, and the power comes back to the victim," NSW Police assistant commissioner Stuart Smith said.
Ms Wormleaton said she's seen other alleged domestic violence cases "thrown out of court because there wasn't enough evidence".
The app let's victims come forward when they are ready to make a report, she said.
"Empower You" is designed for people living in NSW but the "Daisy" app, created by 1800 RESPECT, is available nationwide.
Support is available for those who may be distressed. Phone Lifeline 13 11 14; Men's Referral Service 1300 776 491; Kids Helpline 1800 551 800; beyondblue 1300 224 636; 1800-RESPECT 1800 737 732.