A Kenyan anti-corruption drive uncovered the equivalent of tens of millions of dollars in unexplained wealth when it retired old banknotes, the central bank said on Wednesday, adding much of the cash appeared to have been gained in "the criminal area."

The East African nation has a diverse, fast-growing economy, but foreign investors have long complained of widespread graft and weak application of anti-money laundering laws.

On June 1, the central bank set a Sept. 30 deadline for everyone to convert their old 1,000 shillings note, worth about $10, into new ones after it became the banknote of choice for criminals of all types in the East Africa region.