SWAT uses GBIF (the Global Biodiversity Information Facility) as the source of its weed records. GBIF is a global database of biodiversity information. A wide range of organisations publish to it, including iNaturalist, government and regional agencies, herbaria and other sources.
Anytime you see something new or interesting in nature, you can add an observation to the free iNaturalist platform to share it with the world. Once it is positively identified, it can become research-grade. It will then be published to GBIF within a week or two and will be included in relevant SWAT weed alerts. For example, this can alert councils to new weeds in their region, or weed volunteers to weeds in their reserve.
Note that some observations, such as cultivated plants, will not become research-grade.