Introduction 'Green spaces' such as public parks are regarded as determinants of health, but evidence from tends to be based on cross-sectional designs. This protocol describes a study that will evaluate a large-scale investment in approximately 5280 hectares of green space stretching 27 km north to south in Western Sydney, Australia. Methods and analysis A Geographic Information System was used to identify 7272 participants in the 45 and Up Study baseline data (2006-2008) living within 5 km of the Western Sydney Parklands and some of the features that have been constructed since 2009, such as public access points, advertising billboards, walking and cycle tracks, BBQ stations, and children's playgrounds. These data were linked to information on a range of health and behavioural outcomes, with the second wave of data collection initiated by the Sax Institute in 2012 and expected to be completed by 2015. Multilevel models will be used to analyse potential change in physical activity, weight status, social contacts, mental and cardiometabolic health within a closed sample of residentially stable participants. Comparisons between persons with contrasting proximities to different areas of the Parklands will provide 'treatment' and 'control' groups within a 'quasi-experimental' study design. In line with expectations, baseline results prior to the enhancement of the Western Sydney Parklands indicated virtually no significant differences in the distribution of any of the outcomes with respect to proximity to green space preintervention. Ethics and dissemination Ethical approval was obtained for the 45 and Up Study from the University of New South Wales Human Research Ethics Committee. Ethics approval for this study was obtained from the University of Western Sydney Ethics Committee. Findings will be disseminated through partner organisations (the Western Sydney Parklands and the National Heart Foundation of Australia), as well as to policymakers in parallel with scientific papers and conference presentations.