Umaga's work as defence coach resulted in the Blues topping the Super Rugby Aotearoa statistics for tackles made and number of dominant tackles. He has also contributed significantly to the team culture.
"I am thrilled that Tana will be staying with us. He has done a superb job in the development of this team as a defensive unit," said MacDonald.
"Defence is the cornerstone of our success, not just with the alignment and communication, but our goal-line scramble defence was outstanding in terms of our system and strength of will.
"A big part of that was the character and sense of purpose that Tana has played a major part in developing with this squad."
The coaching group are completing the review of all aspects of the season and have offered support to Northland, North Harbour and Auckland Mitre-10 Cup teams.