Australia's forward stocks have taken another hit with the news that .
"His leadership and influence over our squad has been outstanding to witness - particularly given the challenges we've all recently faced.
"He leaves a real mark on this group with the experience he's been able to impart on the next generation of Waratah and while we're sad to see him go, we understand what a great opportunity this is for Simmo and his young family."
Simmons believes the timing is right to leave Australian rugby.
"It was a really tough decision (to leave) but after speaking with many people around me, I felt that now was the time to take the opportunity to play overseas," he said.
"I've loved my time in Super Rugby and the last few years with the Waratahs, it's a wonderful organisation and it's been an honour to lead this group throughout 2020.
"My team-mates and our management team have been tremendous over the last few years, there's a lot to be excited about here in New South Wales and I'm looking forward to seeing it unfold from afar."
The departure of Simmons is a bitter blow to the Waratahs' rebuilding plans given his vast experience and expertise at the lineout.
While the Waratahs' back-row remains mobile and strong with Test back-rowers Michael Hooper and Jack Dempsey in it, the tight-five has lacked top-notch depth in it in recent years.
Waratahs general manager Tim Rapp said the franchise would have loved to have kept him, but thanked him for his service since arriving in 2018.
"Rob has been a really important signing for us since he arrived in 2018," he said.
"He's been a role model, super consistent in his performance on field and a great ambassador for our game off it - the contribution he's made to our game over more than a decade has been exceptional.
"As much as we'd love Rob to stay, you have to appreciate the opportunity that's been placed in front of him - it's (playing overseas) one of the great things our game has to offer.
"Everyone here at the Waratahs only has the upmost respect for Simmo and we all wish him the very best in the next chapter of his career."
Simmons still qualifies for the Wallabies under the current eligibility laws.
- TEAMtalk media