Milk and growth data from six litters of southern brown bandicoots (Isoodon obesulus) from south-eastern Tasmania were analysed. Five of these were born in the wild and one was born in captivity. The quantitative changes in milk composition were examined for total protein, carbohydrates (hexose and lactose), lipid, dissolved solids, and estimated energy content. During ‘early-phase’ lactation (before Day 30) protein levels were 10.5 g (100 mL)^–1, hexose 0.34 g (100 mL)^–1, lipids 10.4 g (100 mL)^–1, and solids 27.3 g (100 mL)^–1. ‘Mid-phase^’ milk (Days 31–45) showed an increase in all constituents: protein 13.8 g (100 mL)^–1, hexose 1.66 g (100 mL)^–1, lipid 12.2 g (100 mL)^–1, and solids 41.4 g (100 mL)^–1. Protein and hexose levels decreased in ^‘late-phase^’ milk (Day 46–60) to 11.3 g (100 mL)^–1, and 1.58 g (100 mL)^–1, respectively, and the amount of lipids and dissolved solids increased to 26.4 g (100 mL)^–1, and 52.3 g (100 mL)^–1, respectively. There was no evidence of lactose in milk from the latter half (final 28 days) of lactation. Total milk energy increased from 6.0 kJ mL^–1 in early-phase milk, to 8.9 kJ mL^–1 in mid-phase, and 12.0 kJ ml^–1 in late-phase. Changes in body mass, head, tail, and pes lengths were examined for 16 pouch young. Instantaneous relative growth rates were calculated for the final 42 days of lactation, revealing a decrease in overall growth rates from 14.0% increase in bodyweight per day in early-phase, to 7.8% daily during mid-phase, and 3.5% daily increase during late-phase lactation. Analysis of the relationship between milk and growth in I. obesulus demonstrates that morphological and developmental changes, especially the development of endothermy and time of first pouch exit, directly correlate with changes in growth rates, milk energy, and milk composition.