Alleged double killer Russell John Tully had claimed to have been suffering from painful pus-filled boils that inflamed his armpits and groin.
Dermatology experts believe Tully, 48, was most likely suffering from hidradenitis suppurativa when he allegedly walked into Ashburton's Work and Income centre and gunned down three people.
Office workers Peggy Noble, 67, and Leigh Cleveland, 55, died in Monday's shooting.
Lindy Curtis, 43, was critically injured and is recovering in hospital.
Tully had vented frustrations to the Ashburton Guardian about being unable to find a place to live after returning to his hometown.