The New Zealand Ministry of Health (MoH) maintains a number of National Collections, which contain data on diagnoses, procedures and service provision for patients. There are concerns that these collections may underestimate the provision of cancer treatment, but the extent to which this is true is largely unknown. In this brief report, we focus on the Auckland region to illustrate the extent to which the National Collections undercount receipt of surgery in patients with breast, colon or renal cancer, and receipt of chemo- and/or radiotherapy for breast cancer patients with regional extent of disease (all diagnosed 2006-2008). We collected treatment data from the National collections and augmented this with data from Cancer Centres, breast cancer registers, private hospitals and personal clinician databases. The National Collections were used to determine 'baseline' treatment data, and we then compared receipt of treatment to that observed on the augmented dataset. We found that the National Collections undercounted receipt of surgery by 13-19%, and receipt of chemo- or radiotherapy for breast cancer patients by 18% and 16% respectively. Our observations clearly point toward (1) a non-reporting private hospital 'effect' on surgery data completeness; and (2) underreporting of adjuvant therapy to the MoH by service providers.