Didymosphenia geminata (didymo), also known as rock snot is a freshwater diatom (a type of alga). Didymo is currently found in a number of South Island rivers.
The Ministry for Primary Industries has declared the entire South Island a Controlled Area for Didymo. This means waterways are open to recreational activities, but people are legally obliged to prevent the spreading of didymo.
Check, clean, dry
CHECK: Before you leave a river or lake, check items and leave debris at site.
CLEAN: There are several ways to kill didymo.
DRY: Drying will kill didymo, but slightly moist didymo can survive for months.