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.
Magasiva also had roles in the Power Rangers television series, Sione's Wedding and Outrageous Fortune.
Ninja Storm was an important Power Rangers season for me. It was the first one that hit every note I could want in a show. Corny, action-packed, funny, dramatic, witty.
And it all relied on an amazing cast and crew. Gonna miss you Pua Magasiva. Bless you, wherever you are. pic.twitter.com/xURSOJCTW9
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".
In 2015, Magasiva 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".
— Dame Valerie Adams (@ValerieAdams84) May 11, 2019
Television and radio presenter Mike Puru described Magasiva as a good guy.
https://t.co/3TEktVicG3 Sending my love to his friends and family - he would always stop and say hello to anyone that would say hi to him - bloody good guy
Fellow Outrageous Fortune co-star Shane Cortese said he had experienced the honour of seeing the glint in Magasiva's eye "and the sound of his hearty laugh close hand". "My heart goes out to the Magasiva family."
My heart goes out to the Magasiva family and the extended television alumni who had the honor of seeing the glint in his eye and the sound of his hearty laugh close hand. RIP my friend #puamagasiva@ShortlandTweet
On Twitter Scotty Williams wrote: "Absolutely shocked by the news to hear of your passing brother. May your family find comfort at this time. Ia Manuia Lau Malaga... #RIP."
Cassie Roma said: "My friend is gone. My heart is so, so sore. Please, if you need help, reach out. I know it's hard. We can all do hard things. Go well, Pua. Go well."
The death of Pua Magasiva is a gut punch, the way that only these types of deaths can be. The social media tributes read similarly to Greg Boyed... smart, kind, funny, talented, thoughtful... and troubled. A tragedy in every sense. Be kind to each other.
Mary Haddock-Staniland wrote: "RIP Pua, I'm speechless. You were such a delightful, caring, kind and beautiful character. Always had a spring in your step. Much love."
RIP Pua, I’m speechless. You were such a delightful, caring, kind and beautiful character. Always had a spring in your step. Much love @PuaMagasiva ❤️
— Mary Haddock-Staniland (She/Her) (@MaryHadStan) May 11, 2019
A police spokeswoman said there were no suspicious circumstances and the death would be referred to the Coroner.