Inside one of the hermetic retting chambers. Low ceilings and tanked floors (floor is about 600 below the cill) with the remains of pipes going through the walls. The fact that The Railways Department built "the machinery" certainly indicates that heat was an essential component of the process and these tanks were probably filled with plant material and seeped in hot water to separate the woody material from the fibres.
Ashburton Museum is shortly launching a book about the industry; see;
Archives New Zealand [Website search Archway] has a remarkable collection of documents associated with this war time industry. These documents make it clear that the industry struggled on for at least 3 decades after WWII. The collection seems to be quite strong on correspondence and a bit light on engineering, however it is possible to trace the war time establishment of the Linen Flax Industry and ascertain where the factories were. There were 17 factories and one building at Roxburgh which may or may not have been a factory.
The established factories were at; WOODLANDS, GORE, GERALDINE, SEDDON, WINTON, TAPANUI, WASHDYKE, METHVEN, LEESTON, OXFORD, OTAUTAU, BALCLUTHA, MAKIKIHI, WAIKUKU, BLENHEIM, CLYDEVALE and FAIRLIE.
It is also clear that the involvement of NZ Railways (the Railways department?) was to contribute boilers and every plant except one seems to have been fitted with a Class Wf 43 hp boiler. The exception being a single 85 hp Aa Class boiler somewhere yet not determined.