The 21-year-old is the latest young player to sign on as part of the Saints' 'future fest', joining Luke Dunstan, Josh Saunders and Lewis Pierce.
Jack Billings is expected to announce a contract extension in the coming days, with rookies Mav Weller and Eli Templeton also expected to earn senior list contracts.
Dunstan signs up for Saints
Newnes said he had a lot of confidence in the Saints' young list and the group wanted to drive the club back up the ladder in 2015.
After a carving out an important role across half-back and on a wing, he said he was hoping to spend more time in the engine room next season.
"I’m definitely hoping to push more into the midfield," Newnes told the club's website.
"I will have to put a bit of size on again, hopefully a couple of kilos, before we come back and start our running work.
"But becoming an inside midfielder would be ideal."
Rated highly for his work ethic and neat skills, Newnes is also seen as a future leader at St Kilda.
In just his third season, he finished sixth in the Saints' 2014 club champion award.
Head of football Chris Pelchen said Newnes had shown a commitment to improve on and off the field.
"His leadership among the younger players provides a positive example for others to aspire to," Pelchen said.