GEELONG has moved to lock away emerging defender Tom Stewart to a fresh contract after a stellar start to 2018.

The 25-year-old, who is being discussed in All Australian calculations, has extended his 2019 deal until the end of 2021.

The Cats have also confirmed first-year forward Lachie Fogarty and draftee Charlie Constable have extended their initial two-year contracts to 2021.

After finishing fifth in Geelong's best and fairest and awarded best first-year player in 2017, Stewart has continued his rise in a depleted Cats backline this year.

"I'm very grateful to be in the position I'm in, playing at this fantastic club," Stewart said.

"There was absolutely no hesitation to sign and I was very proud when they (the club) came to me with the extra couple of years."

Geelong general manager of football Simon Lloyd said Stewart's leadership had grown in the absence of retirees Tom Lonergan and Andrew Mackie and the injured Lachie Henderson.

"Tom is heavily invested in our football program, consistently driving club standards and endeavouring to be better," Lloyd said.

"He typifies what the club stands for and with his football continuing to grow, challenges himself and the group to go to another level.

"Tom's passion for Geelong as a club and region is genuine, he is committed to giving back to the football community who shaped him as the footballer he is today."

Fogarty played the opening seven games of the season after debuting in round one before he was struck down with groin soreness.

Constable is yet to debut, despite coach Chris Scott admitting he was "excruciatingly close" on a number of occasions after strong performances in the VFL before he was hit with concussion.  

"Lachie and Charlie have demonstrated early in their careers their strong work ethic and their understanding of the game," Lloyd said.

"They both constantly seek to learn from their experienced teammates.

"Lachie has shown he can make a significant contribution at AFL level and Charlie continues to push for AFL selection with consistent form through the midfield in the VFL.

Irishman Mark O'Connor rounded out the quarter of extensions, signing a one-year deal for 2019.  

"Mark's dramatic improvement in his understanding of our code speaks volumes to his work ethic," Lloyd said.

"He has taken a proactive ownership of his career, seeking out coaches and teammates to improve his game. Week upon week Mark has demonstrated the attributes it takes to play at the highest level."

After playing on Easter Monday against Hawthorn, his third AFL match, O'Connor has been a solid contributor in the VFL.

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