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The Herald understands the body of the 38-year-old was found in Wellington this morning.
His brother Robbie, also an actor, has flown to Wellington to be with other family members and Magasiva's widow Lizz Sadler.
A South Pacific Pictures spokeswoman confirmed Magasiva's death and said she could not make any further comment.
Police confirmed they were called to a sudden death at an address in the Wellington central business district in the early hours of this morning.
Magasiva, who has a young daughter from his first marriage, married Sadler in April last year, 18 months after meeting on Instagram. Sadler also has a daughter from a previous relationship.
Magasiva and Sadler travelled to Bali last month to celebrate their first wedding anniversary.
The actor rose to fame on Shortland Street where he played nurse Vinnie Kruse-Miller from 2003-2006, and then returned from 2011-2018.
He co-hosted the breakfast show on Flava, a radio station owned by NZME, but left the show last year. NZME is the publisher of the Herald.
NZME chief executive Michael Boggs said: "It is with deep sadness that the NZME family has today been told of the sudden death of Pua Magasiva. At this incredibly difficult time our thoughts, prayers and condolences go to Pua's family, friends and his many colleagues. For us at NZME, Pua was a loved member of our radio team at Flava. We've been in touch with Pua's former work mates and are offering them support".
Magasiva also had roles in the Power Rangers television series, Sione's Wedding and Outrageous Fortune.
Magasiva was convicted of drink driving on a number of occasions.
In 2015, he split up with his previous wife, Kourtney, whom he had married in 2012.
Tributes are already flowing on social media for Magasiva.
Workers from Flava wrote on Facebook: "RIP to our brother. Our Thoughts and prayers are with his whānau and friends".
R&B artist Vince Harder wrote: "No words uso, praying for your family at this time. Rest in love".
Olympian Dame Valerie Adams tweeted: "Rest In Peace Pua Magasiva. Thoughts and prayers to the family".
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