PURPOSE: Ethnic and socioeconomic disparities exist in treatment of invasive breast cancer in New Zealand. This study investigated trends and disparities in locoregional treatment of ductal carcinoma in situ (DCIS) detected by BreastScreen Aotearoa (BSA), the national breast screening programme. METHODS: Women with programme-detected DCIS from 1999 to 2022 were identified from BSA records linked to the national cancer registry and hospital discharge records. Logistic regression identified associated factors. RESULTS: Of the 6087 cases identified, 39.7 % received breast-conserving surgery (BCS) with radiotherapy (RT), 31.5 % had mastectomy and 28.8 % had BCS alone. BCS with RT increased from 27.6 % in 1999 to 41.1 % in 2006, followed by a modest increase to 46.7 % in 2022, while mastectomy decreased from 33.3 % in 1999 to 25.4 % in 2022. The post-BCS RT use was less common among Pacific women. Sentinel lymph node biopsy (SLNB) increased from 6.2 % in 2004 (when it was implemented nationwide) to 26.0 % in 2007, then reached 37.1 % in 2013, before declining to 24.5 % in 2022. Pacific and Asian women who had mastectomy were more likely to have SLNB. Immediate breast reconstruction (IBR) after a mastectomy increased from 11.4 % in 1999 to 39.8 % in 2009, then underwent a modest decline to 22.7 % in 2022. Māori, older women, and those living in the deprived or rural areas were less likely to receive IBR. CONCLUSION: Locoregional treatment for programme-detected DCIS has improved over time; however, ethnic and socioeconomic disparities persist, underscoring the need to improve equity of cancer care in New Zealand.