Weeds are often grouped in categories depending on their characteristics and impacts. Many weeds occur in more than one category. For example, blackberry is a noxious weed and is also listed as one of Australia's Weeds of National Significance.
The law requires all landholders in certain areas to control certain serious weeds. These are known as noxious weeds, and the law that declares these in NSW is the Noxious Weeds Act 1993.
Under the National Weeds Strategy, 32 introduced plants have been identified as Weeds of National Significance (WONS). A list of 20 was endorsed in 1999 and a further 12 were added in 2012.
These weeds are regarded as the worst weeds in Australia because of their invasiveness, potential for spread, and economic and environmental impacts.
Under the National Weeds Strategy, 28 environmental weeds were identified National Environmental Alert Weeds. Alert Weeds are non-native plant species that are in the early stages of establishment and have the potential to become a significant threat to biodiversity if they are not managed.
Freshwater ecosystems are highly vulnerable to invasion by weeds. Many exotic plants have been accidentally or deliberately introduced into waterways in New South Wales, and have become widespread.
Some Australian native plants have become invasive in areas beyond their natural range or habitat. This is partly due to their popularity as ornamental plants, and partly due to landscape changes. Many are not declared in NSW under the Noxious Weeds Act 1993, but are known to be problematic for land managers.
A number of Australian native plants have been declared on Lord Howe Island under the Noxious Weeds Act 1993. These are species that have been introduced to Lord Howe Island that are not part of the endemic island vegetation communities.
A small number of native plants are declared under the Act in other parts of NSW.