Background: Medication errors in neonatal intensive care units (NICUs) are prevalent, with dosage and prescription errors being the most common. Aims: To identify the common medication errors reported over twelve years using a voluntary, nonanonymous incident reporting system (RiskMan clinical incident reporting information system) at an Australian tertiary NICU. Methods: This was a single-centre cohort study conducted at a tertiary NICU. All medication-related incidents (errors) reported prospectively through the RiskMan online voluntary reporting database from January 2010 to December 2021 were included. The medication incidents were grouped into administration, prescription, pharmacy-related, and others, which included the remaining uncommon incidents. Results: Over the study period, 583 medication errors were reported, including administration-related (41.3%), prescription-related (24.5%), pharmacy-related (10.1%), and other errors (24%). Most incidents were reported by nursing and midwifery staff (77%) and pharmacists (17.5%). Most outcomes were minor or insignificant (98%), with only a few resulting in major or significant harm. There was one extreme incident that may have contributed to the death of a neonate and nine moderate incidents. Conclusions: Our results demonstrate that medication errors are common and highlight the need to support improvement initiatives and implement existing evidence-based interventions in routine practice to minimise medication errors in the NICU.