Pre-sale inspections suggested a "manageable amount" of asbestos had been used to build the pool complex.
Expert advice received last week, however, was that asbestos had been widely used and no-one could safely be on site, Mr Shaw said. Other issues included derelict and hazardous buildings and unsafe walkways.
The company had notified staff and contractors that they may have been exposed to asbestos.
Mr Shaw said the development was "deeply saddening".
"This is a place of significant historic and cultural importance. It's precious to the local community and we really wanted to make something special of it and to bring it up to the standard of other attractions in the region."
Top Energy could not say when, or if, the pools would re-open. The company was still digesting the reports and considering its options, Mr Shaw said.
A security guard was making sure no one entered the site yesterday. The main asbestos
problems are thought to be in the fibrolite changing rooms, but warning signs have also been placed on other buildings around the site.
Fibrolite is widely used around Northland, including Ngawha Springs, in buildings of the same era.
Ngawha Spa's closure does not affect the nearby Waiariki hot pools, which are operated by the Parahirahi C1 Trust and remain open.
Although rather rundown, Ngawha's hot-pool complexes are the main tourist attractions in the Kaikohe area. Ngawha Spa had one of the only campgrounds in the area and featured in the 2004 cult documentary Kaikohe Demolition.
Top Energy was last week granted a raft of consents to expand its nearby geothermal power plant from 25MW to 75MW, subject to conditions.
The consents may yet be appealed. Opponents include the Parahirahi C1 Trust, which is concerned about possible effects on its hot pools.
Ngawha's geothermal field is the biggest in the country after the Central Plateau.
