"It has been great to work with such passionate rugby people - colleagues, volunteers, club coaches, players and many others who contribute to our provincial game.
"The success of the Northland Rugby Union in 2014 is attributed to the collective, so we all should be proud. However, I have been given an opportunity to coach overseas and for many reasons have chosen to accept," he said.
The former Taniwha and All Black hooker previously assisted Adriaan Ferris in the 2011 and 2012 seasons and was part of the push to use more local players in the squad - partly because of financial constraints but also to retain a committed squad core. The success of the 2014 season was also underlined by a greater number of super rugby contracts awarded to Taniwha players.
His loss is a double blow for the Taniwha with 2013-14 assistant coach George Konia also resigning in December to take up another coaching post.
Witcombe told the Northern Advocate yesterday he was unable to say at present where he would be coaching other than to say it was overseas.
"I would like to thank the ITM Cup management team and players; we've worked hard together to create what we have and I wish the Taniwha every success in the future. I will always actively support the union no matter my location," he said.
The chief executive of the NRU, Jeremy Parkinson, said Witcombe's loss would be keenly felt. "Unfortunately, it's just the nature of the business we're in.
"The ITM Cup is a stepping stone to other professional rugby competitions, be it the Super 15 or overseas, and it's no real surprise that Derren's been made an offer."
He said while the timing of his resignation was hardly ideal, the union had given their blessing.
"The positive for us is that we've had four good years of outstanding service from him, two as assistant coach and two as head coach.
"He's left a great legacy behind him particularly in the way he's championed local players and that leaves a solid platform for his replacement, he's a good Northland boy and the doors always open for him."
Parkinson said they would go to the market immediately to look for a replacement with applications closing in three weeks.
