The 35-year-old said his retirement from representative football in 2018 was a big factor in his decision.
"I was feeling really good after the 2018 season, given it was my first time in a long time away from the representative arena," Smith said on Thursday.
"Finishing with no injuries ... I'm feeling as good as I've ever felt and that was the reason I asked the club for two years and, thankfully, that was an option that worked for them as well.
"If it was a deadset 'no' from the club then I would have been happy to go ahead with the 12 months."
