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Prattley, who will turn 23 next month, played for Tasman in this year's ITM Cup, although he came off the bench in some matches.
He played for the New Zealand under-20 side in 2010 and debuted for Tasman in the same year. He has the ability to play both sides of the scrum and was a member of the Crusaders' wider training squad last season.
Although the Highlanders do not play a pre-season game until February 2, they are facing a minor propping crisis.
Chris King has had a shoulder operation and is on limited training, while Ma'afu Fia is also not 100 % fit after surgery on his thumb.
With star new signing All Black Tony Woodcock not joining the team until the start of February, last year's captain, Jamie Mackintosh, is the only fit prop in the 31-man squad.
King and Fia are scheduled to be back and available by the start of the season but Prattley has been called in to cover and will be an option should they fail to recover in time.
King struggled with a groin injury for the majority of last season.
Experienced prop Bronson Murray, who has been a member of the Highlanders for the past two seasons, is a member of the team's wider training squad.