It is important to explore the types of conceptualisations and causes presented in online mental health promotion given the implications that these presentations may have on mental health stigma. This study systematically reviewed 92 Australian webpages focused on either mental health, mental illness, depression, or schizophrenia, to explore the types of conceptualisations and aetiologies presented. A minority of mental health and mental illness webpages (n = 8, 8.70%) explicitly presented continuum conceptualisations, with none providing explicit categorical conceptualisations. No depression or schizophrenia webpages presented explicit conceptualisations of any kind. All four webpage foci had a greater proportion of continuum than categorical conceptualisations. Moreover, both depression and schizophrenia webpages presented many mixed conceptualisations which included both continuum and categorical messaging. Most webpages mentioned biological and social causes equally across webpage foci. These findings suggest that Australian mental health websites predominantly present continuum conceptualisations of mental health and mental illness.