"This is hugely disappointing for everyone, but we must, and will, put staff and public safety first. We simply have no choice," he said.
Council staff have begun working with groups that have booked the theatre to find new venues. They include Hamilton Operatic staff and performers who were rehearsing for Mary Poppins.
"We have already been in contact with Hamilton Operatic and many others and we're working with those groups individually to see what can be done. Staff are working as hard as they can to help make alternative arrangements."
Where possible, new venues will be advised as soon as they are confirmed on the Ticketek website or at founderstheatre.co.nz although that may take some days. Ticket holders should hold onto their tickets until new venues are confirmed.
Briggs said he accepts the closure was sudden but says it was well-known Founders Theatre needed major work to bring it up to a standard where it could host modern performances. Work has been done on the flying system in 2010 and 2014 to extend its life.
"The staff have done an incredible job to keep it going. But Founders is now 54 years old and band-aid solutions won't work anymore. Council has already allocated $16.4m in its 10 Year Plan for a major refurbishment but this is going to require a re-think of what comes next."
A comprehensive report will go to council later this month outlining potential options.
Briggs said council is working through the legal implications of closing but acknowledges the council might be liable for some costs associated with bookings.
"Our prime focus up until now has been around safety concerns."