Hector's dolphins are polygynandrous, as both sexes have multiple mates. Males search for receptive females rather than monopolizing access to individual females and aggressiveness is not associated with sexual behavior. Mature males have very large testis relative to their body size, making up as much as 2.9% of total body mass.

Sexual or social behaviors (e.g., presenting one's abdomen or penis and physical contact) are often associated with mating. Hector’s dolphins exhibit increased sexual behavior (per individual per minute) in larger groups then in smaller groups. Sexual behavior is 2 times greater in groups of 11 to 15 individuals than in groups numbering 1 to 5 or 6 to 10, and nearly 20 times higher than in groups of 16 to 20 individuals. The presence of young does not appear to affect sexual behavior.

Mating System: polygynandrous (promiscuous)

Male Hector’s dolphins reach sexual maturity between ages 6 and 9, and females reach sexual maturity between ages 7 and 9. They mate in the summer, have a gestation period between 10 and 12 months, and parturition occurs from early November to mid February. Hector’s dolphins reproduce every 2 to 4 years and usually one calf is born at a time. Females can give birth to a maximum of 7 calves during their lifetime.

Breeding interval: Hector's dolphins reproduce every 2 to 4 years.

Breeding season: Hector's dolphins breed during the austral summer

Range number of offspring: 1 (high) .

Average number of offspring: 1.

Range gestation period: 10 to 12 months.

Range weaning age: 12 to 24 months.

Range time to independence: 1 to 2 years.

Range age at sexual or reproductive maturity (female): 7 to 9 years.

Range age at sexual or reproductive maturity (male): 6 to 9 years.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); viviparous

Average number of offspring: 1.

Mother and calf stay together for 1 to 2 years, during which time the mother does not breed. Females with calves often separate from non-calving individuals and form larger calf-cow groups. Once they reach 2 years old, calves become more independent and have been observed in groups with other juveniles and no adults.

Parental Investment: precocial ; female parental care ; pre-weaning/fledging (Protecting: Female); pre-independence (Protecting: Female); extended period of juvenile learning