The Gavin & Stacey actress claimed that the show undermines professional performers who have worked in the industry for many years.
Talking to The Stage, the 34-year-old actress said: "There are actors and musical theatre performers who have given their whole lives to the profession because it is their passion, and then you have some bloody van driver on a show [like Superstar] saying 'I didn't do this when I was 12 but I wish I had done, but I didn't have the guts'.
"Well, that's your problem, because you obviously didn't have the passion, spirit, drive and the determination to dedicate your whole life to it."
She added: "I am finding myself really happy now that they are left with people who aren't suitable. There is no-one with the charisma, the authority, the stature or the maturity to play Jesus."
Page also took umbrage with the way Lord Webber apparently neglected performers who have worked for him in the past, saying: "Think about Ramin [Karimloo]. He was in a rock band and he is amazing. He would even look like Jesus.
"But you have Andrew Lloyd Webber saying about a U2 song Rory [Taylor] sang - which was performed with no facial expression because he's dead in the eyes - that he did not know any musical theatre star who could sing like that.
"What about Ramin? He just played the Phantom for you. How insulting. It's disgusting."
The Welsh actress also spoke of her issues with the celebrity casting in the final production - "Mel C is judging who will be Jesus, but how can she then have a working relationship with whoever wins?" - and the depiction of the audition process - "How am I going to believe in the Jesus I am watching when I have seen him in some video with his mum and dad, with his mum showing us baby photos and the contestant messing around with his niece?"
"I could not give a s**t if he loves his niece," she added. "I just want to see Jesus."
