They've been in place for 25 years but it seems Australia's mandatory bike helmet laws are still stoking passions and stirring controversy.

The helmet laws have emerged as the surprise focus of federal Parliament's so-called "nanny state" inquiry.

Senator David Leyonhjelm at Parliament House. Credit:Andrew Meares

Set up by libertarian senator David Leyonhjelm​, the inquiry is examining a wide array of laws and regulations that may be seen to unnecessarily impinge on people's freedoms.

"I thought the Sydney lock-out laws would be the big one," Senator Leyonhjelm said. "I didn't think we would get swamped by the helmets."