BackgroundIn 2009, the New Zealand government introduced a hospital emergency department (ED) target â 95% of patients seen, treated or discharged within 6Â h - in order to alleviate crowding in public hospital EDs. While these targets were largely met by 2012, research suggests that such targets can be met without corresponding overall reductions in ED length-of-stay (LOS). Our research explores whether the NZ ED time target actually reduced ED LOS, and if so, how and when.MethodsWe adopted a mixed-methods approach with integration of data sources. After selecting four hospitals as case study sites, we collected all ED utilisation data for the period 2006 to 2012. ED LOS data was derived in two forms-reported ED LOS, and total ED LOS - which included time spent in short-stay units. This data was used to identify changes in the length of ED stay, and describe the timing of these changes to these indicators. Sixty-eight semi-structured interviews and two surveys of hospital clinicians and managers were conducted between 2011 and 2013. This data was then explored to identify factors that could account for ED LOS changes and their timing.ResultsReported ED LOS reduced in all sites after the introduction of the target, and continued to reduce in 2011 and 2012. However, total ED LOS only decreased from 2008 to 2010, and did not reduce further in any hospital. Increased use of short-stay units largely accounted for these differences. Interview and survey data showed changes to improve patient flow were introduced in the early implementation period, whereas increased ED resources, better information systems to monitor target performance, and leadership and social marketing strategies mainly took throughout 2011 and 2012 when total ED LOS was not reducing.ConclusionsWhile the ED target clearly stimulated improvements in patient flow, our analysis also questions the value of ED targets as a long term approach. Increased use of short-stay units suggests that the target became less effective in âstanding forâ improved timeliness of hospital care in response to increasing acute demand. As such, the overall challenges in managing demand for acute and urgent care in New Zealand hospitals remain.Electronic supplementary materialThe online version of this article (10.1186/s12913-017-2617-1) contains supplementary material, which is available to authorized users.