Taxes to be paid after increase and backpay have been worked out

I have a very basic question, but cannot wrap my head around it. I pay the admin ladies' salaries (for about last year). They got an increase, but I need to backpay them now as well. Question is does the annual salary (March 2015 to Feb 2016) now need to be recalculated to see what more tax must be paid? Is there someone that can help me on an annual basis? I am so scared I work in out wrong. Please help!

Yes, you would need to recalculate the annual salary for Feb 2015 - Feb 2016 (including the back pay) and then recalculate the taxes due over this period. This will ensure the correct tax is paid for the tax year and the IRP5 will correctly reflect the income and related taxes for the year.