"It's a big commitment, but everyone's very excited about visiting various community theatres," he says.
The spark for the new musical was lit while Webber was on a family holiday in London 12 years ago. He found himself waiting for his wife and children at the foot of Sir Christopher Wren's Monument to the Great Fire of London, less than a minute away from Pudding Lane where the fire broke out.
"I instantly knew it would be a terrific idea for a musical," Webber recalls, "and I could see the settings and had the characters and music in mind but I came home and, well, you fall back into where you were at before, don't you?"
Not quite; the idea didn't completely vanish. He started reading and researching the Great Fire, slowly writing the script and composing the music. The diaries of Samuel Pepys, kept from 1660-1669, became favourite reading - "I could be up to two or three in the morning reading those" - as did histories of the fire itself and the English Restoration.
Hundreds and hundreds of pages were written; many were discarded until Webber thought he had it just right. The show was performed at the Rose Centre in what he and Thorpe describe as more of a workshop season.
It's certainly a sizzling moment in history to recreate: a time of massive upheaval, Charles II restored to the Monarchy after hard-line puritan Oliver Cromwell's 11-year rule and a city recovering from a devastating plague outbreak.
Webber starts the story with a boy and a girl meeting at the historic Queenhithe Market by the River Thames as London recovers from the deadly plague, but there's worse to come: a long spell of hot dry weather has seen the City of London become tinder dry.
Freelance musician George Bonner leads a live band, who plays musical numbers ranging from harpsichord period pieces to rock 'n' roll, and there are 22 actors and singers aged from their teens to their seventies.
Fire on the River includes 30 musical numbers and follows the spread of the fire from its start at midnight on September 2 at Farriner's Bakery until it was finally extinguished on Wednesday, September 5.
"I learned so much! I didn't know the extent of what Oliver Cromwell and his Puritans had done to the theatre by closing them all down and I was fascinated to discover a French watchmaker [Robert Hubert] confessed to starting the blaze and was hung at Tyburn.
"I couldn't help thinking it was a sign, given my background as a watchmaker, that I was on the right track."
But let's hope Webber is luckier than Hubert; it was later found that Hubert couldn't possibly be responsible for the fire because he was at sea when it started.
What: Fire on the River
Where & when: Playhouse Theatre, Glen Eden, October 6 - 16; Spotlight Theatre, Papatoetoe, October 18 - 23; Pumphouse Theatre, Takapuna, October 25 - 30.