These are the elements of an offence: something is prohibited, the commission of which shall lead to consequences (usually) such as a fine or imprisonment or both. That, in essence, is what a statutory provision purporting to outlaw any act should do.
Ask yourself this: what is the penalty for using, manufacturing or importing plastic carrier and flat bags on or after the 28th August? Is possession of plastic bags synonymous with use, manufacture or import?
I have read the Environmental Management and Co-ordination Act, 1999 carefully. It creates 51 offences, but it is section 144 that should interest you:
Any person who contravenes against any provision of this Act or of regulations made thereunder for which no other penalty is specifically provided is liable, upon conviction, to imprisonment for a term of not less than one year but not more than four years, or to a fine of not less than two million shillings but not more than four million shillings, or to both such fine and imprisonment.
"Ah ha!" I hear some of you saying. The plastic ban says that the Cabinet Secretary has banned the "use, manufacture and import" of plastic bags so if one uses, manufactures or imports plastic bags, one commits an offence. QED! It is not quite so simple: the Cabinet Secretary's Notice is not a regulation at all because it does none of the things contemplated by section 86 of the Act (the provision, presumably, under which the Notice was published). Therefore, contravening the provisions of the "ban" is not an offence under the general penalty contained in section 144.
Look at it from this perspective: the Act does not give the Cabinet Secretary the power to ban anything. Therefore, she cannot ban the use, manufacture or import of plastics. Therefore, her "ban" is unenforceable. Therefore, there is no ban. Therefore, using, manufacturing or importing plastic bags is not an offence. Therefore, if one does use, manufacture or import plastic bags, one shall not be liable to "imprisonment for a term of not less than one year but not more than four years, or to a fine of not less than two million shillings but not more than four million shillings, or to both such fine and imprisonment".