Aberrations to the early gut microbiome have been repeatedly associated with later-life disease. While previous studies have explored the assembly of the infant gut microbiome, none have done so in an exclusively breastfed population, nor in an Australian context. Here, we characterised the temporal development of the breastfed infant gut microbiome over the first 12 months of life in the BLOSOM birth cohort using full-length 16S rRNA gene sequencing. Exclusive breastfeeding was a strong determinant of the infant gut microbiome, with introduction of solid food driving a rapid and dramatic expansion in the gut community. Richness and Shannon diversity remained stable over the first 5 months of life, followed by a significant increase in diversity from 6 to 12 months (P<0.0001). Bifidobacterium was the dominant genera in this cohort, with Bifidobacterium longum supsp. infantis representing the most abundant species. Bifidobacterium relative abundance increased across the first 4 months of life from a mean relative abundance of 23% at 2-5 days to 46% at 4 months. Bifidobacterium levels then began to steadily decline from 5 to 12 months (12 month mean relative abundance = 33%). The persistence of this health-promoting genera a 12 months of age is likely a reflection of the high breastfeeding retention rates in the BLOSOM cohort (>80% still breastfeeding at 12 months of age). C-section delivery was a major determinant of the infant gut microbiome, with significant impacts on Bifidobacteria and Bacteroides. Similar impacts were seen in vaginally delivered infants whose mothers were exposed to intrapartum antibiotic prophylaxis, suggesting that antibiotic exposure related to delivery mode, rather than delivery mode per se, is responsible for the effect. In this first study of gut microbiome assembly in exclusively breastfed Australian infants, we demonstrate a high abundance of Bifidobacterium that persists to 12 months of age. Our results also demonstrate the dramatic impact of introduction of solid foods, and exposure to intra-partum antibiotics. These data illustrate “optimal” infant gut microbiome development associated with vaginal delivery and breastfeeding.