The Kiwi outfit has struggled in the opening nine rounds, recording just two wins to sit bottom of the Hyundai A-League.
The two-time A-League champion felt that with recent results he had taken the team as far as he could.
“I take full responsibility for position the team is currently in,” Merrick said.
“I feel that I have not performed well enough as Head Coach, as we have a very strong squad that should be higher up the table.
“It’s therefore the right time for me to step aside and allow someone else to take over. We have a run of home games coming up and I think this will give my successor the best opportunity to take the team forward.
“I’d like to thank everyone at Wellington Phoenix for the past three years and particularly Welnix, the Chairman and the board who have been nothing but supportive during this time.”
The club understands the decision and thanked Merrick for the hard work and dedication he has put in during the past three years.
Wellington Phoenix have appointed Chris Greenacre and Des Buckingham as joint Interim Head Coaches while the club undergoes a thorough process to fill the role full time.
Football Federation Australia