New Zealand locked down nursing homes nationwide Wednesday after a 102-day streak without the coronavirus ended, as Prime Minister Jacinda Ardern said the outbreak could force her to postpone next month's general election.

Ardern said authorities were scrambling to trace anyone who had been in contact with four Auckland residents who tested positive Tuesday, ending the dream run in which the virus had been contained at New Zealand's borders.

A three-day stay-at-home order for Auckland, New Zealand's biggest city with a population of 1.5 million, was announced on Tuesday night and went into force at lunchtime on Wednesday.

Android phones will receive warnings triggered by a "ShakeAlert" earthquake early-warning system implemented on the West Coast by the US Geological Survey and partners.

ShakeAlert uses signals from hundreds of seismometers across the state to trigger warning messages that "an earthquake has begun and shaking is imminent," according to the system's website.

"We saw an opportunity to use Android to provide people with timely, helpful earthquake information when they search, as well as a few seconds warning to get themselves and their loved ones to safety if needed," principal software engineer Marc Stogaitis said in a blog post.