Carlton are still weighing up who to select with the first pick of the 2018 National Draft, as AFL draft expert Callum Twomey believes midfielder Sam Walsh isn't locked in to go at No. 1.

Walsh has been the consensus number one selection for the majority of the second half of the season, although South Australian gun Izak Rankine is looming as a potential shock pick.

“I’m not 100 per cent sure on (Sam Walsh) being a lock at pick one,” Twomey told SEN Breakfast.

“I think Izak Rankine’s sort of come with a little bit of a rush and they’ve been considering him late in the piece.

“I think by now they would’ve come to a decision.”

Both South Australian clubs are try to work their way up the draft order as high as they can go, with Port Adelaide currently having selections 5 and 10, while Adelaide hold picks 8, 13 and 16.

Rankine is still expected to be the first SA player to be selected, although Twomey believes the Blues will stick with Walsh.

“If I had to put my house on it, I’d say Sam Walsh is the guy and the odds are still in his favour, but the attraction with Izak Rankine is really obvious because he’s a match-winner,” Twomey said.

“He’s probably someone you can’t find in a draft in the next few years.

“Sam Walsh, as good as he is, he’s a hard-working midfielder with real class, but you can probably find them in the next couple of years.

“That’s where I think they’ll be weighing things up, the Blues.”