Barrett, 20, was initially named in the under-20 squad last month but with his hot form for the Wellington franchise and injuries in the Super Rugby team, he has been left out of the final roster.
Blues first-five Stephen Perofeta has also been ruled out due to his ongoing recovery from an ankle injury.
The New Zealand under-20 coach Craig Philpott said he is happy with the depth in his squad.
"With Stephen and Jordie not available we needed extra cover at first-five-eight. Ciarahn Matoe's time with the Chiefs under-20 squad has gone well and he proved himself with the New Zealand Secondary Schools team last year. This is a good opportunity for him and one I am sure he will make the most of," said Philpott.
"We had hoped that Stephen would be available for selection. However, it's become clear he won't be ready in time for the tournament due to his ankle injury."
The squad will travel to the Gold Coast for matches against Fiji on April 28, Samoa on May 2 and Australia on May 6. All matches will be played at Bond University, Gold Coast Rugby Fields.
NZ under-20 squad:
Forwards:
Harrison Allan - Canterbury
Asafo Aumua - Wellington
Sam Caird - Waikato
Adrian Choat - Auckland
Tom Christie - Canterbury
Ryan Coxon - Waikato
Tim Farrell - Hawkes Bay
Alex Fidow - Wellington
Luke Jacobson (C) - Waikato
Ezekiel Lindenmuth - Auckland
Marino Mikaele-Tuu - Hawkes Bay
Dalton Papalii - Auckland
Jacob Pierce - Auckland
John (JP) Sauni - Auckland
Samuel Slade - Auckland
Pouri Rakete-Stones - Hawkes Bay
Isaia Walker-Leawere - Wellington
Backs:
Caleb Clarke - Auckland
Ereatara Enari (VC) - Canterbury
Braydon Ennor - Canterbury
Tima Faingaanuku - Tasman
Tiaan Falcon - Hawkes Bay
Will Jordan - Tasman
Kemara Hauiti-Parapara - Wellington
Orbyn Leger - Counties
Ciarahn Matoe - Taranaki
Josh McKay - Canterbury
Jona Nareki - Otago
Carlos Price - Wellington
Tamati Tua - Northland
Thomas Umaga-Jensen - Wellington
