Guest, 57, currently starring in the musical Wicked, was admitted to Melbourne`s St Vincent`s Hospital last night, a spokeswoman for the production said.
Guest is best known for playing the Phantom in Phantom of the Opera for most of the 1990s in Australia.
Wicked producer John Frost said the cast from Wicked, in which he plays the Wonderful Wizard of Oz, were "shocked".
"Rob Guest is one of Australian music theatre`s biggest stars: there have been few bigger in our lifetime," Frost said in a statement.
"He is, nonetheless, the most easy-going of stars. He may have top billing but at all times he comes across to us - producers, fellow cast members and backstage crew - as an irrepressible, always cheerful optimist.
"An all round good bloke. All of us who know him are shocked."
It`s understood his family has flown from New Zealand to be with him.
The English born star who started his career as a pop star in New Zealand, is the world`s longest serving Phantom with a record 2289 performances over seven years.
More recently he played Captain Von Trapp in The Sound of Music. He`s also starred in productions of Les Miserables and hosted the short-lived 90s TV game show Man O`Man.
FANS of British rockers Oasis can hear the band`s new album in full on MySpace tonight, four days before its official release
FILLING the shoes of John Travolta is a tough task for anyone, and Guy Sebastian was feeling the pressure while trying to channel his best Danny Zuko impression yesterday
HEATH Ledger`s haunting Joker has been voted as the second greatest movie character of all time behind Indiana Jones, in a new poll
KELLY Osbourne gets tested for sexually transmitted infections every three months, she has revealed
