Online book retailer Booktopia expects to increase stock by 25 per cent and cut delivery times when a $4.5 million investment in robotics goes live later this year.

Booktopia co-founder and chief executive Tony Nash said the robotic “shelf stackers”, which stack books onto four-metre-high shelves, would enable the e-tailer to increase the stock in its Lidcombe warehouse in Sydney by 200,000 to 1 million.