In many cases, the water supply company will simply need a description, diagram and plan of the work, the contact details of who is authorising it and the details of the plumber/plumbing business being used. More details can be found on the WaterSafe website.
If you use a WaterSafe approved plumbing business, they are able to carry out some types of work without notification. This is because they have specific training in the strict regulations required to install pipes and fittings to supply drinking water.
Getting permission is quick and free, but failure to notify your water supply company could result in extra costs to put poor plumbing right or, worse, contamination of water supplies and a court prosecution.