BACKGROUND: The New South Wales (NSW) and Australian Capital Territory (ACT) Cerebral Palsy (CP) Register is a database of clinical and demographic information from children with CP. A child with CP resulting from an insult to the developing brain sustained between 29 days and 2 years of age is classified as having post-neonatally acquired CP (PNN-CP). In clinical services, children may meet the criteria and timing for PNN-CP but have a singular diagnosis of acquired/traumatic brain injury. OBJECTIVES: To implement and evaluate a new CP register ascertainment strategy focused on identifying children with PNN-CP attending acquired brain injury rehabilitation services. METHODS: Electronic medical records of children with an acquired brain injury attending the Sydney Children's Hospitals Network and John Hunter Hospital rehabilitation departments 2019-2024 were reviewed by researchers and rehabilitation paediatricians to identify children with PNN-CP. Children who fulfilled the criteria for CP were invited to participate in the CP Register. To evaluate this ascertainment strategy, we (i) ran descriptive statistics to analyse proportional changes of children with PNN-CP on the register and (ii) calculated temporal trends in prevalence per 10,000 live births for birth years 2003-2016, before and after the record ascertainment period. RESULTS: Of 1051 children with an acquired brain injury, 46 had PNN-CP (2003-2019) and had not previously been included on the register. This ascertainment strategy resulted in increased prevalence of PNN-CP in all 2-year time points between 2003 and 2016 and equated to a 31% improvement in ascertainment of children with PNN-CP on the register. CONCLUSIONS: Ascertainment of children with PNN-CP for the NSW/ACT CP Register has been improved by systematically reviewing children with an acquired brain injury. This is now part of standard practice, and other registers should consider whether this strategy may improve ascertainment of PNN-CP in their regions.