RIVAL clubs are circling North Melbourne free agent Robbie Tarrant after the former forward's strong start to 2015 in defence.

Injuries have limited Tarrant to 46 games since he was taken by North with pick No.15 in the 2007 national draft, but the 26-year-old has been a revelation since shifting to the backline this season.

Most clubs have been watching closely as Tarrant, 196cm and 97kg, has held his own this year against elite power forwards such as Tom Hawkins, Jarryd Roughead and Travis Cloke.

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North recently opened preliminary contract discussions with Tarrant in the hope of re-signing him before he becomes an unrestricted free agent at the end of this season.

But AFL.com.au understands that at least two rival clubs have signalled their preparedness to offer Tarrant up to $400,000 a season to lure him away from Arden Street.

Suitors will be keen to see Tarrant play a full 2015 season given his injury-prone history, but he has become a very attractive commodity given the paucity of key defenders in the market and his ability to play in attack.

The Brisbane Lions and Western Bulldogs are two teams actively in the market for key-position players, while Carlton, Fremantle, Adelaide, St Kilda and Geelong are among a raft of other clubs investigating trade and free agency options to shore up their spines.

Rival list managers told AFL.com.au it would be reasonable for North to offer Tarrant about $250,000-$300,000 a season plus bonuses to play on at Arden Street.

All agreed he could attract significantly more from opposition clubs, with some saying offers of up $450,000-$500,000 a year were possible.

Clubs considering giving Tarrant a big pay hike will be emboldened by the widely held expectation that the salary cap will be significantly boosted from 2017 when the next broadcast and collective bargaining agreements come into effect.

"In 2017, $450,000 or $500,000 a season is not going to take up as much of your TPP (total player payments) as it does now, so clubs will look at making offers around this mark for Tarrant," one list manager said.

"There are not a lot of tall defenders in the market either and, unlike a draftee who needs years of development, Tarrant is ready to go.

"Put it this way, he's not getting any cheaper for North Melbourne at the moment."

The key defender played a straight bat when AFL.com.au asked him about his contract status last month.

"I'm just really enjoying my footy at the moment. I'm sure my manager and the club will speak at some stage and get something done," he said.

Tarrant has bounced back this season from recurring tibia stress injuries that restricted him to one senior game last year and sidelined him for nine straight matches in 2013.

He did not earn a senior call-up this year until round three after the Roos preferred fellow key defender Joel Tippett in their opening two games.

But the former Bendigo Pioneer has scarcely looked back since coming into the Roos' team against Port Adelaide in round three.

North has lacked a 'gorilla' key defender in recent seasons but Tarrant has shown enough in his seven games this year to suggest he could develop into a very good one.

The former forward has also retained some of his attacking instincts in his new role, averaging 17.3 possessions (ranked 10th at North) and three rebound 50s (third) this year.