A Sydney council has warned companies operating dockless share bikes in the city to ensure they're locked up or face fines.

Reddy Go's red bicycles and oBike's yellow machines have sprung up across Sydney and Melbourne this year but critics say the bikes are too often being dumped in public spaces or vandalised.

An illegally dumped Obike in Sydney's Darling Harbour precinct. Credit:Les Hewitt

Randwick City Council believes the problem is the lack of secure parking for the dockless bikes, which can be left anywhere after they're used.

Councillor Harry Stavrinos' motion that council demand the operators provide secure parking was backed at Tuesday night's council meeting.