Screening for severe combined immunodeficiency (SCID) was added to the New Zealand national newborn screening programme in December 2017. Documentation pertaining to the application to add SCID to the panel and screening results over the first three years were reviewed. Screening evaluation metrics were shown to differ according to site of collection (babies in a neonatal intensive care unit vs. the community), definition of a positive test (out-of-range result vs. result leading to a further action on baby), and screening target/case definition (primary SCID vs. non-SCID T-cell lymphopenia). Our experience demonstrates both the value of close clinical involvement during the implementation phase of SCID screening and that the use of standard definitions will facilitate international comparison.