How does one go about applying to be taxed on a cash basis? This is for normal income tax.

My wife is an occupational therapist and the nature of her work is that payments can come a number of months later (sometimes even years). AFAIK in SA the general principal is that we work on an invoice basis for taxation, but it is possible to apply to be taxed on a cash basis.

How does one go about applying for that?

(I haven't checked out SARS website yet, will do shortly and report back if no one has the answer)