Mauger and fellow councillor and supporter James Daniels said the work fixed the flooding overnight.
But the council, which plugged the trench to stop it working, questioned its effectiveness and said the trench created health and safety and environmental risks.
A council spokesman said on Wednesday the regulatory compliance unit had completed its investigation into the incident and issued Mauger with an infringement notice and fined him $300.
Mauger also agreed to do free maintenance work in the red zone, at no cost to the council, and has since been repairing a walkway and cycleway under the northern side of New Brighton’s Anzac Bridge.