An Uber-style bus proposal for Adelaide has been criticised by union officials.

The service involves passengers requesting a bus through an app and being picked up and taken to their location along an undesignated route, but the Transport Workers Union said the service will be unpredictable due to multiple pickups increasing travel time.

The South Australian government invited proposals to trial the “demand responsive” service in areas with lower bus use as a part of an overhaul of the public transport network.