Mooar was part of former coach Scott Robertson's coaching group before Mooar stepped up to the All Blacks coaching group under Ian Foster. However, that proved short-lived, and Mooar moved offshore to work with Scotland.
But Ellison's elevation created a chance to bring Mooar back into the Crusaders, a move welcomed by coach Rob Penney.
"Brad, having four years here, knows the place intimately, but then having the experience of coaching offshore with the Scarlets and then having a relationship with the Scottish international side really rounds you off as a coach.
"Getting some international exposure, and then getting all that experience coming back into the office, we're very fortunate and rapt to have him in."
Mooar and James Marshall will work with the Crusaders' backline in specific areas.
Mooar said he had kept in touch with contacts in Christchurch, and the opportunity arose to be involved with the Crusaders again.
It was also ideal from a family perspective.
"This is our team, this is home and to be back at Rugby Park with some wonderful people that I've worked with before, and some new faces as well, to build some new relationships."
Mooar said his and fellow assistant coach John Plumtree's departure from Ian Foster's coaching team had been challenging. However, learning from all experiences was important, along with how you respond.
"It didn't dampen enthusiasm for coaching or leading and growing.
"You just take stock, reflect, and go, 'OK, what would be the right thing next?'
"I've learned long enough to know that we say good experience, bad experience, take the good and bad away and just treat things as experiences and deal with them as they come.
"What do we learn from, and how can we grow from that and get better?
"I don't hold any grudges whatsoever around that. It was a great privilege to be involved with the All Blacks, I learnt plenty and had some great experiences. You're right at the sharp end of high performance sport and international rugby. It was a privilege."
Mooar said his motivation was seeing people 'rip the ceiling out of potential' and drive themselves to work together in a team with the desire to try and be better each day. That was the same mantra in rugby, at school, in the workplace and in the corporate world.
"That really drives me and I'm passionate about and love the sport."
Penney said that after a harrowing first season with the Crusaders missing out on the playoffs action for the first time in eight years, it was clear that no one person had all the answers.
"We had some challenges last year, which, hopefully, we've learned and adapted from and Brad coming in with a fresh set of eyes, and the experience, also has the intellectual capacity to challenge anybody, which is a good thing."