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.

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).

They helped develop the 1989 Children, Young Persons and Their Families Act which adopted a radical, family-centred approach to the secondary prevention of child maltreatment.

He was a co-founder, with Robin Fancourt, Judy Bailey and Dame Lesley Max, of the BrainWave Trust which lobbies policy-makers on behalf of children.

With Jocelyn Cowern, Allan Barber and Andrew Davidson he co-founded the Kids Help Foundation Trust which operates the 'What's Up' national helpline for children and young people.