Labour says an Otago University study linking overcrowding with infectious diseases reinforces the need for more affordable housing.
Researchers reviewed 10,000 published papers to find crowding is a direct cause of one in 10 hospital admissions in New Zealand for illnesses such as pneumonia, tuberculosis and meningococcal disease.
The team also found large ethnic inequalities.
Labour housing spokesperson Phil Twyford said the study painted a shocking picture that must be remedied.