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Act's sole MP was walking towards the court's entrance when an elderly man stepped forward and threw a bucket load of mud at him, spattering his face and suit.
Penny Bright, who was protesting against Banks outside the court, witnessed the incident and said the man "just biffed it all over" Banks.
"He just looked like thunder," she said of Banks, who then composed himself and walked inside.
Ms Bright said the man, who was wearing a purple polar fleece top and grey track pants, was angry about something Banks had said on Radio Pacific in 1997.
Court security were dealing with the man.
Banks is alleged to have knowingly received political donations from internet entrepreneur Kim Dotcom and SkyCity that were recorded in official returns as anonymous during his bid for the Auckland mayoralty in 2010.
The allegations relate to two $25,000 donations from Dotcom and a $15,000 donation from SkyCity.
Banks' trial will be heard by Justice Edwin Wylie, without a jury.
It was due to start at 10am but will now begin at 11.30am.