‘Australian wormy chestnut’ is a marketing name. One commercial website advises that a parcel of wormy chestnut could include any or all of the following: Eucalyptus obliqua (messmate), Eucalyptus sieberi (silvertop ash) and Eucalyptus fastigata (brownbarrel).

WoodSolutions is an advisory body and we don't supply or install timber products. The only company we know of that produces hardwood timber shakes is Australian Hardwood Shingles & Shakes, website available via this link https://www.shinglesandshakes.com.au/. We are not sure if they also handle installation. If not, no doubt they have contacts in the industry and can recommend experienced contractors.

To confirm the level of treatment, samples can be sent away to a testing lab where they can be assessed against the requirements of the Australian Standard. For example, IVS Labs provide this service, website here https://www.ivslabs.com.au/. There are two aspects to be tested, penetration (depth of preservative) and retention (amount of preservative in the penetration zone). Sampling is more straightforward when the full cross-section of the piece is accessible. If it's impractical to analyse the full cross-section of the piece, eg.

It is correct that Appendix F of Australian Standard 1604.1:2021 recommends H4 treatment for fence plinths and other ground contact situations. Appendix F is "informative", ie. for information and guidance only. However Table 1.5.2, in the performance part of the Standard, lists a similar requirement and H4 treatment for timber in ground contact can be taken to be an integral part of AS 1604.1. While H3 treated plinths are not a risk to public safety, nevertheless they don't comply with Australian Standard product performance requirements.

Unfortunately no timber is classed as 'non-combustible' when tested to Australian Standard 1530 Part 1: Combustibility test for materials. The test involves placing specimens in a furnace at approximately 750°C, at which temperature they must produce only a limited amount of heat and flame.