A spiralling oversupply of shared bikes in China is leading to huge piles of broken and unused bicycles in cities across the country.

Over the past year, about 30 companies have madly pumped out millions of new bicycles from factories, distributing them in major centres in a battle for market share.

The two biggest companies, Ofo and Mobike, claim to collectively have 20 million users registered with their services.

The bicycle boom has revived cycling in a country that was once affectionately referred to as the bicycle kingdom, prior to three decades of high-speed economic growth that flooded the streets with cars.

But now there is growing concern about a bubble, as big investors rush to pour money into bike sharing.

The key appeal of the shared bikes — that they do not need to be returned to a dock — is proving problematic in busy spots, where bikes are piling up and blocking pedestrians.

Broken and discarded shared bikes pile up in a Beijing backstreet. ( ABC News: Bill Birtles )

As new bikes roll off production lines, workers for Ofo are busy fixing tens of thousands of twisted and broken bikes piled up for almost a kilometre on a Beijing backstreet.

One worker told the ABC he fixes about 20 bikes a day, which are then placed on trucks and unloaded near subway stations around the city.

Another company, Bluegogo, is hoping to bring the bikes to Sydney, as a starting point to roll them out to other Australian cities.

"We're not asking to dump tens of thousands of bikes on the streets of any city," Bluegogo's chief operating officer Ye Sun said.

"What we're asking for is: can we do a trial program, put bikes there to see if people like them?"

Mr Sun concedes mandatory helmet laws in Australia pose a challenge.

The City of Sydney is undertaking a feasibility study due to be completed mid-year.

In a statement, Mayor Clover Moore acknowledged mandatory helmet laws were partly responsible for a low uptake of docked bike-sharing schemes in other Australian cities.