Helmets take a hike from Melbourne oBikes

Users of a Melbourne bike-sharing service are being left without head protection with almost half the helmets reported missing.

The oBike rental scheme, in which registered users can pick up and drop off bikes across the city, started in June and has already lost about 40 per cent of its helmets, which are meant to remain attached to the bicycles.

The company estimates an average non-return rate of up to 20 per cent and the scale of helmet loss in Melbourne is a "unique Australian issue", spokesman Chethan Rangaswamy told AAP.