Studies indicate that age-standardised heart failure (HF) incidence has been decreasing internationally; however contrasting trends in different age groups have been reported, with rates increasing in younger people and decreasing in elderly. We aimed to describe age- and sex-specific trends in HF incidence and aetiology in New Zealand (NZ). In this nationwide data-linkage study, we used routinely collected hospitalisation data to identify incident HF hospitalisations in NZ residents aged ≥20 years, between 2006 and 2018. HF aetiology was categorised as ischaemic or non-ischaemic. Age-specific and age-standardised incidence rates were calculated for each calendar year. Joinpoint regression analysis was used to compare trends by age group, sex, and aetiology. 116,113 incident HF hospitalisations were identified over the 13-year study period. Between 2006 and 2013, age-standardised incidence decreased from 403 to 323 per 100,000 (annual percentage change [APC], -2.6%; 95% CI, -3.6% to -1.6%). This reduction then plateaued between 2013 and 2018 (APC, 0.8%; 95% CI, -0.8% to 2.5%). The proportion of heart failure admissions associated with ischaemic heart disease has decreased from 35.1% in 2006 to 28.0% in 2018. Whilst the incidence of ischaemic heart failure is either falling (≥60 years) or stable (<60 years), the incidence of non-ischaemic heart failure is increasing in people <70 years. Heart failure remains an important problem in NZ. The decline in overall incidence has plateaued since 2013 due to increasing rates of non-ischaemic HF in younger age groups despite an ongoing decline in ischaemic HF across all age groups.