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The event, at the Corinthian Conventions Centre in Invercargill, gathered 238 projects from students who worked in pairs, trios or groups of four.
Environmental education officer Pat Hoffman said it was a great opportunity for the children.
''It's pretty amazing what these kids can do.''
A prizegiving ceremony will be held on Friday, students being recognised in different subjects.
Pictured, Tom Collie (10) of Limehills School discusses his ''Clean or Dirty Creek'' entry with judges Kate Holdsworth (left) and Urte Bierlin.