Data on book sales for Amazon Australia, the major e-commerce-only vendor, is not available. But growth in online sales relative to in-store is expected to continue along with trends in the retail sector more generally.

"Online retail is the growth bright spot in Australian retail. It is the segment of retail in growth," said Bain & Company partner Melanie Sanders, who leads the firm's local retail practice. "On average, it's growing about six times the rate of bricks and mortar retail."