Wesfarmers-owned Coles is testing its first online-only store in inner-city Melbourne as it pushes to drive volumes in online sales, which have jumped 25 per cent this year alone.

Coles boss John Durkan said the chain planned to roll out one or two more of the so-called "dark stores", which have a dedicated team picking stock for online customers.

Mr Durkan said the idea was to test if Coles could become more efficient in handling online shopping and avoid clogging up the supermarket aisles with the bulky online order trolleys.

The online-only outlet was opened in April at a former Coles site in Richmond, and serves customers living within about five kilometres, taking in some of Melbourne's most densely populated suburbs.