Posting on Facebook under his real name Gerard Sayer, the singer commented on a review of McCartney's performance, saying: "Embarrassing. Hasn't anybody got the bottle to tell him 'Mate sorry, we all love you, but the voice is gone..."
Sayer, 64, now an Australian citizen, fired up again after music website Noise 11 posted a story on McCartney and other artists including British star Emile Sande being paid one pound to perform at the Olympics.
"In the case of Emile Sande, they should've given her a decent fee," Sayer wrote. "In the case of McCartney, they certainly got their money's worth, didn't they.."
Sayer toured Australia in 2006 on the Let It Be tour singing Beatles hits written by McCartney and John Lennon.
At the time Sayer said "I have had the honour of recording half a dozen Beatles songs in my comparatively short career and every time I sing those songs I realise how great their talent was. Without the Beatles the world would be a sadder place. I knew most of the guys personally, being lucky enough to have met them when I was starting out on my career. They all gave me advice like rock n roll parents and I think I've become a better artist for knowing them."
Sayer has covered Beatles songs including Let It Be, The Long and Winding Road and I Am the Walrus.
Sayer plays charity show Duets in Sydney on August 20, he then tours the UK in November on a retro bill with David Cassidy, Hot Chocolate and Smokie.
McCartney, meanwhile, has been recording with Amy Winehouse producer Mark Ronson in London over the last few weeks.
"We worked on a few songs, I think it's his new album," Ronson told News Limited.
"It's all super secretive. It feels like Mission Impossible, you just turn up to the studio and do good work and you're not really told what it's for. That's fine."