GII.b norovirus is an obligatory recombinant with an open reading frame (ORF) 1 comprising the GII.b genotype and the ORF 2 region corresponding to one of a number of other norovirus genotypes. GII.b is the second most common genotype after GII.4 associated with gastroenteritis outbreaks in Victoria, Australia. The study involved norovirus testing of 14,186 specimens from 2,743 Victorian gastroenteritis outbreaks in the period 2002-2011. The noroviruses identified were further characterized by nucleotide sequencing and phylogenetic analysis. In the period 2002-2005, 21 GII.b norovirus outbreaks were identified, with the GII.b ORF 2 recombinant genotypes comprising GII.1 (47.6%), GII.3 (47.6%), and GII.13 (4.8%). For the period 2006-2011, 56 GII.b norovirus outbreaks were identified. In 51 of these, the ORF 2 genotype was identified, and comprised GII.1 (2.0%), GII.3 (94.1%), GII.13 (2.0%), and GII.21 (2.0%). GII.b norovirus outbreaks could occur in a range of settings involving individuals with a broad range of ages. However, GII.b/GII.3 norovirus tended to occur in a younger demographic and all outbreaks involving specifically children's settings had the GII.b/GII.3 genotype. Nucleotide sequencing studies demonstrated major changes in both the ORF 1 and ORF 2 regions of the GII.b/GII.3 noroviruses but not in the GII.b/GII.1 and GII.b/GII.13 noroviruses when the noroviruses for the two time periods 2002-2005 and 2006-2011 were compared. The findings suggest nucleotide changes in the pre-existing GII.b/GII.3 noroviruses resulted in a more virulent form.