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Dermody will share the duties with Andrew Goodman, who was a foundation player for the Tasman side when the Mako started in 2006.
The duo take over from Leon MacDonald, who has moved on to be an assistant coach at the Blues.
Dermody worked with the Tasman forwards this year but has won elevation to the main coaching role with Goodman.
Former Highlanders flanker Shane Christie, who was involved in the coaching group this year, has been promoted to forwards coach.
Christie was forced to retire from the game due to concussion-related issues in May this year.
Tasman lost to Canterbury in the semifinals of the Mitre 10 Cup this year.