Most of the literature review of wireless sensor actuator networks (WSANs) focused on single level collision avoidance while accessing a medium. In this paper, we try to solve the issues that arise after completing a particular stage of medium access. This paper executes collision avoidance in WSANs for cyber physical system (CPS) based smart wireless networks (CPS-SWNs). We tried to improvise the IEEE 802.15.4 MAC protocol to handle the successive collisions that occur between any two base stations. Since we are concentrating more on CPS applications like smart offices, smart homes, we implemented a medium access control methodology referred as priority with counter modified backoff (PCMB) that suits for non-beacon enabled WSANs. A priority with counter is maintained at each device to avoid collisions in unslotted 802.15.4 protocol. Further, we have proved from the simulation results that successive collisions can be delayed rather than providing data loss in critical situations. © Copyright 2016 Inderscience Enterprises Ltd.