The Historical Aircraft Restoration Society building at the Illawarra Regional Airport will be closed to the public for about eight weeks to allow for the completion of internal works.
"It is regrettable but necessary from a safety point of view," HARS president Bob De La Hunty said.
He said systems to deal with fire safety in the building would be installed during the closure.
A construction certificate allowing the program of works to improve fire safety would be delivered to the council by Tuesday morning.
Mr De La Hunty said the closure meant a training organisation - Aerospace Training Services (ATS) - that uses the facility would need to make other arrangements.
He said there would be no impact on staff associated with the region's aeromedical helicopter, which is stored in the HARS hangar, as they were not considered "general public".
Mr De La Hunty said visitors were signed in as guests visiting a construction site but this would not be possible with the scale of work about to take place.
The closure comes after Shellharbour City Council threatened HARS with "regulatory action" because the building, various parts of which had been occupied since 2005, did not have an occupation certificate.
In April, Shellharbour councillor Peter Moran raised concerns over public safety after the council had confirmed that no occupation certificate had been issued on the building.
The council blamed the "disjointed" and "often unco-ordinated" construction of the various stages of the building as the reason for the lack of an occupation certificate. But they said expert advice had considered the fire risk to be "reasonable".
However, concerns were heightened earlier this month when Cr Moran revealed that commercial entities were also present in the council-owned building without the council's knowledge.
The council had set an August deadline for HARS to present the construction certificate for safety works, warning that a notice of intention would be served requiring the occupants to cease using the premises.
In a report to be presented at Tuesday night's council meeting, council staff said HARS was permitted to sublease parts of the building, under the agreement with the council.
The council was working with HARS to "regularise" all sub-leases at the facility, the report said.
The arrangement with ATS was a "verbal agreement" that was not formally documented.
"ATS do not pay rent to HARS and they make a contribution to outgoings," the report said.
Concerns were also raised about the presence of Historical Aircraft Restorations Pty Ltd.
However, HARS chairman Michael Hough said the entity was linked to HARS and it did not make money.
A full report on subleases at the facility would be presented to council later.