Bankserv have indicated that the problem was with the ACS servers of ABSA and potentially FNB as well.
This issue now appears to have been resolved.
We will contact merchants with affected transactions if any further action is required from them.
Please do not hesitate to contact us on firstname.lastname@example.org for any questions that you may have.
We apologize for any inconvenience that may have been caused by this.
Your Peach Payments Team