The objective of this study was to estimate breed and heterosis effects between Holstein-Friesian (F) and Jersey (J) cows at different herd production levels in herds milked once-a-day (OAD) or twice-a-day (TAD) in New Zealand. Three groups of herd production levels based on milksolids (MS, fat + protein) production per cow were considered: low, medium and high. The average MS production per cow was: 203.6; 269.2 and 339.9 kg of MS in herds milked OAD; and 272.7; 353.8 and 434.1 kg of MS in herds milked TAD. Data consisted of 322,327 lactation records from 35,192 F; 31,118 J and 88,606 crossbred (F×J) cows that calved in spring between 2008 and 2012. Breed effects, defined as F-J, increased as production level of the herd increased in both systems. Heterosis effects, expressed as a percentage of the mean of the parental breeds, ranged between 3.3 and 8.4% in OAD and 4.4 and 7.4% in TAD systems. The highest expressions of heterosis were found at medium (6.3-8.4%) and high (6.1-7.4%) production level in cows milked OAD and TAD, respectively. In conclusion, production level affects the expression of breed and heterosis in both milking systems. Breed and heterosis effects increased as production levels increased.