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This paper reports on both the individual and combined effects of age (AFC) and liveweight (LWFC) at first calving for Australian Holstein–Friesian heifers on first lactation production. One hundred and thirty-five Australian Holstein–Friesian heifers were allocated to 1 of 3 AFC treatments. Within each AFC treatment, heifers were randomly assigned to 1 of 3 LWFC treatments. Heifers in all groups grazed pasture and were supplemented when the quantity and quality of pasture was inadequate to meet growth requirements. Mean AFC and LWFC achieved were 25.1, 29.9 and 33.9 months and 498, 549 and 595 kg, respectively. Mean liveweight gains from 16 weeks of age to calving ranged from 0.45 to 0.71 kg/day, depending on treatment. The heifers calving at 33.9 months of age produced 6.6 and 12.3% more milk, 6.3 and 11.9% more protein and 5.4 and 12.2% more fat than those calving at 29.9 and 25.1 months of age at the end of their first 300 day lactation, respectively. The lower production of the younger cows was associated with decreased daily output rather than by shorter lactation length. Heifers averaging 595 kg at first calving produced 5.5% more milk, 8.4% more protein and 11.4% more fat than those averaging 498 kg in first lactation, respectively. The heifers averaging 621 kg LWFC and 34 months AFC had the highest production of the 9 treatment groups. Production was increased by 5.35 L milk, 0.19 kg protein and 0.23 kg fat for an additional 1 kg LWFC, respectively. For each month delay in AFC, production was increased by 66.7 L milk, 1.87 kg protein and 2.36 kg fat, respectively. The combined effects of AFC and LWFC showed that to offset the negative effects of a 1 month reduction in AFC on milk, protein and fat yields in first lactation, LWFC would have to be increased by 8.1, 4.0 and 4.5 kg, respectively. Under the conditions of this experiment, maximum milk, protein and fat were estimated to be achieved at 559, 563 and 568 kg liveweight at first calving, respectively.