In a bombshell move by Rugby Australia, New Zealander Rennie was peremptorily fired after three seasons in the post and replaced by Australian Eddie Jones, who was sacked as England coach in December.
In a statement Rennie expressed disappointment that he wouldn't be able to fulfil his contract and lead them to this year's Rugby World Cup.
"I'm disappointed I won't be able to see out my contract in the way I agreed to back in 2019 but leave knowing I had the full support of the playing group and the staff."
"I certainly felt we have made massive shifts over the past three years both on and off the field, which is off the back of a hell of a lot of hard work put in by good people.
"I wish Eddie, the staff and the team all the best in what's a massive year, with the Rugby World Cup less than nine months away."
Rennie's demise was ascribed to his dismal 38% win record in his time in charge, which included two losses to Jones's England in a three-match series last July.
"I've loved my time with the team," the 59-year-old added.
"They're outstanding young men who are keen to learn and prepared to work hard.
"The staff I worked with during my time with the Wallabies are some of the best in the world and they played a massive role in creating a quality environment and developing the depth of the playing group."
Australian media reports have linked Rennie with a move to Japanese club Kobe Steelers for next year.