CCA (copper-chromium-arsenate) Treated timber sleepers are treated to H4 level for in ground use and should last up to 40 years and often longer once installed. H4 treatment has been used as a timber treatment for about 70 years and is suitable for in ground and above ground use, in direct weather. Other treatment levels are available upon request, such as H5 for fresh water positions and H6 for salt water conditions. H4 is not intended to be used in water although often is.
The treatment type used is classified as envelope treated, meaning it was treated to a minimum of between 2mm to 10mm deep. The centre of the timber is not treated, so care must be taken if cutting.
The issue with termites occurs after the timber has been cut, exposing the heart wood and then used in ground. Such as for pergola posts, and specifically retaining walls.
Buying a spray on or brush able treatment to treat cut ends will assist to maintain the protection, although this is rarely done.
The retaining wall that is built by most house owners, if under 1000mm high requires no council development approval and therefore no engineering in most cases.
Most are built without the required treatment to cut ends and unfortunately are quite expensive to repair.
This can however usually be identified by a building and pest inspector prior to you purchasing the property, if you have it inspected.