New Zealand has many long viaducts constructed back when seismic design provisions were not accounted for during design and construction phases. A study of such structures can present a unique dataset for the researchers for understanding the behaviour of these bridges that are on critical transportation networks. This research aims to capture the key parameters controlling response of the eighty-two year old Whirokino Viaduct that spans one kilometre over the Manawatu River floodplain. 20 triaxial accelerometers were installed along its length during two recording periods to collect ambient vibration data, first when the bridge was fully intact, and second with several spans at the southern end removed. This data will allow for an assessment of the effects of the abutment on the dynamic response. Data was analysed using the Modal Parameters Identification Toolbox developed at The University of Auckland to identify the dynamic characteristics such as natural frequencies and mode shapes. Also, a computational model of the bridge was developed using OpenSeesPy to represent the elastic characteristics of superstructure and substructure components. The obtained mode shapes and natural period were compared with that from sensor data and the influence of a range of structural and foundation system parameters was assessed in order to identify the key parameters controlling the dynamic characteristics of the Viaduct. It is hoped that the outcomes of this study will improve the understanding of the in-service response of bridges and inform design assumptions related to the contribution of different bridge components to overall dynamic response