SOUTH Australian defender Harrison Petty shapes as a possible surprise in the first 25 picks at this month's NAB AFL Draft as clubs look to bolster their key position stocks.

The 194cm backman enjoyed a strong 2017 season, and was named South Australia's most valuable player at the NAB AFL Under-18 Championships. He also won All Australian honours for his efforts at the carnival.

Some clubs rate Petty as one of the best tall defenders available at the draft, with his strong overhead presence and good rebound making him a contender to break into the latter stages of the first round or early in the second round when names are called.

Part of Petty's appeal is the view from various recruiters that he is relatively untapped, with this season his first in the elite talent pathway after coming from a regional background.

The 17-year-old averaged 12 disposals and four marks at the national carnival but did not complete any of the athletic testing at the national NAB AFL Draft Combine after rolling his ankle in the kicking and goalkicking trials on the opening night.

With the majority of the early part of this year's draft dominated by midfielders and smaller types, the key position prospects are more difficult to place with a wide variety of opinions on where they sit in the pecking order.

West Australian pair Aaron Naughton and Oscar Allen are expected to feature in the first round, while Bendigo Pioneers swing man Jarrod Brander is also likely to be picked in the same range. Powerful South Australian forward Darcy Fogarty is another who has been linked as a possible top-10 choice.

Tom McCartin, the younger brother of St Kilda's Paddy, has suitors and could be selected inside the first 30 picks after showing promising signs at stages this season, while defenders Oscar Clavarino and Sam Taylor are also a chance to fit in the same bracket.

Aiden Bonar's progress late in the year after returning from two knee reconstructions has rocketed him into first-round calculations, and the 190cm forward has also been linked as a possible top-10 pick on draft night. However, that appears more unlikely than likely.