Background: Interval breast cancers (BC) are those diagnosed within 24 months of a negative mammogram. This study estimates the odds of being diagnosed with high-severity BC among screen-detected, interval, and other symptom-detected BC (no screening history within 2 years); and explores factors associated with being diagnosed with interval BC. Methods: Telephone interviews and self-administered questionnaires were conducted among women (n = 3,326) diagnosed with BC in 2010–2013 in Queensland. Respondents were categorised into screen-detected, interval, and other symptom-detected BCs. Data were analysed using logistic regressions with multiple imputation. Results: Compared with screen-detected BC, interval BC had higher odds of late-stage (OR = 3.50, 2.9–4.3), high-grade (OR = 2.36, 1.9–2.9) and triple-negative cancers (OR = 2.55, 1.9–3.5). Compared with other symptom-detected BC, interval BC had lower odds of late stage (OR = 0.75, 0.6–0.9), but higher odds of triple-negative cancers (OR = 1.68, 1.2–2.3). Among women who had a negative mammogram (n = 2,145), 69.8% were diagnosed at their next mammogram, while 30.2% were diagnosed with an interval cancer. Those with an interval cancer were more likely to have healthy weight (OR = 1.37, 1.1–1.7), received hormone replacement therapy (2–10 years: OR = 1.33, 1.0–1.7; > 10 years: OR = 1.55, 1.1–2.2), conducted monthly breast self-examinations (BSE) (OR = 1.66, 1.2–2.3) and had previous mammogram in a public facility (OR = 1.52, 1.2–2.0). Conclusion: These results highlight the benefits of screening even among those with an interval cancer. Women-conducted BSE were more likely to have interval BC which may reflect their increased ability to notice symptoms between screening intervals.