Dave Rennie has been backed by the man he replaced as Wallabies coach after his furious criticism of match officials' decisions during the Test defeat to Wales.

Michael Cheika said he wanted to offer his support to his successor Rennie, whose outburst after Saturday's (Sunday morning AEDT) 29-28 defeat in Cardiff may yet earn him punishment from World Rugby.

After hearing Rennie describe some of the officiating at the Principality Stadium as "horrendous", Cheika, in his role as a pitch side pundit for Prime Video, said he understood the frustration.

"I've been right there, right where he's standing, feeling just like that, and I want to give him my support right now," Cheika, the former world coach of the year who steered Australia to the 2015 World Cup final but was replaced by Rennie in 2019 after five years in the job, said.

Rennie was infuriated by numerous decisions in Cardiff, particularly by what he saw as the inconsistency of Kurtley Beale being sin-binned for a deliberate knock-on while Wales centre Nick Tompkins was allowed to go on and score despite a similar incident.

TMO Marius Jonker was the focus of Rennie's wrath as he asked why the South African official had been allowed to officiate just couple of weeks after another of his rulings in the Wallabies' defeat by Scotland - the sin-binning of Allan Alaalatoa - had later been ruled incorrect by World Rugby.

"Coach Rennie is pointing out that incident (with Tomkins) but I think it's the accumulation (of rulings), also the involvement of the TMO in many decisions, and he'll be feeling frustrated because, just like me, he felt his side had played really well and gritty...," Cheika said.

"Like he (Rennie) said, they deserved more on the night. He'll be feeling that now, so let's give him a bit of room to vent and tomorrow he'll be sweet."

Rennie was uncharacteristically angry, saying it was the first time in more than 20 years as a professional coach he'd felt the need to castigate match officials in public, and reckoned he was neither over-reacting nor emotional.

Yet his attack, particularly on an experienced official like Jonker, left him risking a reprimand from World Rugby.

The governing body had earlier in the week given South Africa's director of rugby Rassie Erasmus a two-month ban for publicly criticising Australian referee Nic Berry over his performance in the first Springboks-Lions Test.

Rennie himself said afterwards that he believed Berry hadn't deserved to be treated that way.

As for Australia's performance with 14 men - and for 10 minutes, 13 men - in Cardiff, Cheika felt they could take a lot out of their last match of 2021, despite returning home as the first Wallabies for 45 years not to win a Test on their annual European jaunt.

"That was their best performance of the tour and they played some really good attacking rugby... even though they'll be very disappointed, they should be really proud of themselves and the effort they made in a hostile environment," he said.