The International Scale of River Difficulty is a standardized scale used to rate the safety of a stretch of river, or a single rapid. The grade reflects the technical difficulty, skill level required, and dangers associated with the section of river. Specifically, the grading system for New Zealand and Australia is as follows:

Grade 1: flat water. generally unobstructed passage.
Grade 2: Small to medium sized waves and a few obstacles.
Grade 3: Fairly high waves, and the passage may be difficult to recognize from on the river.
Grade 4: High, powerful, irregular waves, with the passage often difficult to recognize and negotiate.
Grade 5: Very difficult rapids, High-powerful-irregular waves, difficult to negotiate, the extreme for commercial operations.