The new deal keeps the exciting young leader at the Saints until at least the end of the 2019 season.
Newnes enjoyed a career year best season in 2016, playing every match and averaging 23.3 disposals per game and kicking 15 goals.
The 23-year-old said he had no hesitation in extending his stay at the Saints.
“My manager, Peter Lenton, came to me and gave me the offer from the club. We were pretty happy to sign it straight away, so I’m really excited (to sign) for another couple (of years),” Newnes said.
“I like to get [contracts] out of the way early, it takes a bit of pressure off. I just like to worry about my football.
“I’m happy to lock in for three more years and hopefully we can start something special.”
While he has already become a valuable member of St Kilda’s line-up, Newnes says he can continue to improve.
“I think each year I’ve been really taking steps forward. It’s been really fun,” Newnes said.
“I just want to keep taking my game to the next level, hopefully keep improving along with all the other boys.”
Newnes has played 91 games since being selected with pick No.37 in the 2011 National Draft.