The SPCA has advised the public not to bring cats to its Mangere shelter until further notice.
There were between 140 and 150 cats at the shelter before the rescue operation, the largest on record.
Ms Midgen described the case as "good intentions gone haywire".
Despite the "unbelievable, abominable" living conditions, she said the cats were in a better way than first thought.
Some needed medical attention, but there were no life-threatening conditions.
The SPCA is carrying out an investigation into the case, which could lead to a prosecution.
The investigation will determine the future of the cats.
The shelter is unable to accept any cats for at least a week, perhaps longer.
"The decision to halt cat admissions is not taken lightly. We simply have no space left," Ms Midgen said.
She said the SPCA would happily provide people with help and advice on finding a new home for cats.