The announcement was made on the band's social media this week with a video by all four band members giving the news to fans and introducing "the new guy".
Trembath said leaving was a decision that he'd been considering for quite a long time.
''I have really struggled with touring and being away from home so much, and I know that it's only going to get more intense as the band grows, so I've decided to focus my energies on getting work in a recording studio or the production side of things, as that's what I've enjoyed most during my time with Alien Weaponry,'' he said.
''It's been an awesome experience, and I know we'll always remain good friends. And I really want to thank our fans all over the world for their amazing support, which has been truly humbling."
The band has been riding high in the global metal scene, having played the main stages at a number of the biggest metal festivals in Europe and America in 2018 and 2019.
"A world tour that took in 21 countries and spanned seven months was pretty punishing," singer/guitarist Lewis de Jong said.
"We played to literally hundreds of thousands of fans, meeting many of them in person and sharing the stage with some of our all time favourite bands like Lamb of God, Gojira and Slayer."
He said Trembath is one of the original Alien Weaponry crew and the band will miss his massive presence.
''But we get it, we have to respect his wish to spend more time with family and have a home life. The thing is, he lives just up the road so it's not really goodbye. We fully expect to see a lot of Ethan on our visits back home to Waipū, New Zealand," Lewis de Jong said.
Henry de Jong said: "Tūranga was in my class in school and we had some mean times together playing music. We ran auditions with 10 different bass players and in the end it came back to our home boy from Bream Bay College. We are stoked to have him with us as we gear up for the next phase in our journey. Ethan has been a beast helping with the audition process and fully getting into choosing his replacement. We love him like a brother and he will always be part of the AW family."
Morgan-Edmonds praised good mate Trembath for his support.
"He has been so engaged and supportive during the handover and I feel privileged to be picking things up where he left off. Ethan is one of metal's most energetic and charismatic bass players and I know he will be a hard act to follow."
The band are working on their second album in which Trembath has played a big part. Newcomer Morgan-Edmonds will also feature on the album, adding his voice to some of the background vocal parts. The band have been tracking vocals ready for mixing and mastering in their studio in Waipū.