CCA Stands for Copper Chromium Arsenate.
This treatment was designed specifically to protect timber from the attack of insects such as termites as well as the decay of fungus like dry rot etc.

It has been more successful with soft wood such as Radiata Pine because softwood is what we call an open grain timber, where as hardwood is a closed grain timber.

All timber when growing has two types of graded wood, there is a band of Heartwood and a band of Sapwood, now with the open grain timbers (softwood) has a larger band of sapwood (the outer part of the timber) and a much smaller diameter core (heartwood).

This allows the treatment process to succeed with better result to that of hardwood, which has a larger portion of heartwood. Although it is still recognised as being effective in treating hardwood it proven to be better with Radiata Pine than any other, it also has had a bad success rating in treating Oregon (Douglas Fir), Cypress Pine (which has its own oil that protects it from termites), and Bunya Pine to name a few. Because this form of treatment is not guaranteed with certain types of timber, it is illegal to treat them with CCA.

It is rated by using a Hazard Level system and this is easily detected by a H1, up to a H5 label attached to the timber on purchase. Hi being protected but must be above ground and protected from the elements, like house frames etc. H5 can be placed in ground and exposed to the elements.

The way in which the timber is treated is called VPI, this stands for Vacuum Pressure Impregnation. Special treatment plants operate where the timber is impregnated with the mix of CCA, once the treatment has penetrated the Sapwood then it remains in the timber and is highly resistant to leaking out.

Because of the process in which the treatment is ministered it is more successful at protecting the timber than what you could achieve by using stains or paint.

There has been a lot of controversy with this method of protection in regards to its long term effect on the environment so much so that the rules with its usage were changed in 2006. And they are as follows. Although these rules Do Not apply to existing structures they do however apply to all new construction work that uses this timber from that date onwards.
It is still ok to use the CCA Treated Timber for the construction of the following:

All kinds of Fencing.
All kinds of Retaining Walls.
All kinds of frame work for Decking.
All the structural timber for the construction of houses etc. This includes Battening, Fascia’s, Cladding, and such like.
Pergola’s and gazebo’s
Utility Poles and Cross-Arms.

On the other hand there are now several structures that we are No Longer allowed to use treated pine for and these are as follows:

Garden furniture, domestic or commercial.
Picnic Tables.
External Seating.
Any kind of children’s Play Equipment.
Decking Boards for domestic jobs.
Handrails, except those that are primed and carry the structural timber rating.
Other Points to consider.

The treatment process on the timbers such as Radiata Pine is done at a time when the timber is still very young, so its nearly all Sapwood. The treatment will not change the characteristics of the timber itself, It’s STILL PINE and will twist, warp, buckle, split and shrink. So be wary of this when planning a job and you need to use it.
It is still advisable to protect the timber even though its treated, against the elements, either painting, oiling or staining the timber will increase its durability with the Weather, UV Rays, The extreme Heat and Cold, and continue to do what you have used it for.
Remember it’s not 100% guaranteed against termite attack. There are too many other factors involved but CCA is the best form of treatment we currently have.