 In 1978 Hassall was appointed Deputy Medical Director of the Plunket Society to help develop a project that brought a higher level of service and support to South Auckland, which was then widely seen as a disadvantaged area.Notable additional projects in which he took a leading role were, the 'Stamp Out Measles', 'Fencing of Swimming Pools'and Plunket-in-Neighbourhoods' campaigns.He was a member of the research team led by Professor Ed Mitchell which provided the first convincing evidence that prone lying was a major risk factor for Sudden Unexpected Death in Infancy (SUDI).