The work was being carried out under warranty at no cost to the Eden Park Trust Board, Mr Kennedy said.
"The remedial work is of a minor nature and affects secondary steel work that is not structurally important."
It is understood the work has been going on for at least two months at the cost of tens of thousands of dollars and is far from finished.
An engineering expert with experience in building stadiums, who asked not to be identified as he was not aware of the specific facts relating to Eden Park, said the amount of time it took for a structure to begin to rust depended on the surface treatment applied to the steel.
Most stadiums would be built with a 5-, 10- or 20-year maintenance guarantee.
Mr Kennedy indicated the present work would fully fix the problem.
Fletcher Building representatives did not respond to phone and email messages requesting comment.
A report tabled at a July meeting of Auckland Council's finance and performance committee indicated the stadium's finances allowed for paying interest repayments and operational costs but there was no capacity to "reduce debt further [or] address major maintenance issues".
The trust board will also borrow $1.8 million for an extended sound barrier around the Eastern Stand.
The problem
• Workers have spent months treating rust that has appeared on the structure supporting the sound shell on the redeveloped South Stand
• Cost estimated in the tens of thousands of dollars, to be met by Fletcher Construction as stadium is still under warranty
• Eden Park chief says work is routine, minor and cosmetic