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The 20-year-old has made the New Zealand under-20 side for the past two years and will replace the injured James Lentjes.
Lentjes had a shoulder operation last month and is scheduled to get back on the field in April.
Tu'u is seen as a No8 who can also play on the blindside flank so it not a straightforward swap for Lentjes.
Highlanders coach Aaron Mauger said he was comfortable with the cover he had on the openside flank with Dillon Hunt and Dan Pryor both capable of wearing the No7 jersey.
He said Tu'u had impressed them with his ball-carrying ability, had nice footwork and was a player of considerable promise.
It was an opportunity to get Tu'u down into the camp and have a good look at him.
Elliot Dixon was also capable of playing openside flank but the former All Black was on the sideline at the moment after an operation to fix a damaged bicep. He was scheduled to play again in mid-February.
Dixon has been replaced in the squad by loose forward Phil Halder, who impressed for Southland. Halder (24), like Tu'u, is yet to play Super Rugby.