The register, established in 2008 under the Rudd Government, was promoted as an effort to bring transparency to the lobbying industry, particularly in light of the "WA Inc" scandals involving former West Australian premier Brian Burke.

But the register and its associated code of conduct only covered third-party lobbyists and Australian National University Professor John Warhurst told Fairfax Media that at most it covered a third of all lobbyists working the corridors of power.

Other groups, from unions and industry associations to in-house government relations staff at major companies, religious entities, non-government organisations and charities, were exempted on the grounds it was clear who they were acting for.

"If (the purpose) is to give a broad picture of the amount of lobbying and who's lobbying on what and for whom, then it has to include a wider range of lobbyists," Professor Warhurst said.

"Otherwise you've got a lobbying register that only covers a third of all the lobbyists in the country.

"The only justification for it not being extended is that it's administratively too hard, but these days I really don't think that argument stands up."

Figures from May 2015 on "sponsored" car park passes – which are largely issued to lobbyists of varying shades – shows as many as 1497 people have access to federal decision-makers, but it is unclear how many of those are actually registered.

Senator Xenophon said while he didn't necessarily support widening the registers remit, he believed a fulsome review was "long overdue".

"It is well overdue, but it needs to be done in a credible way, and I think it should be part of a broader review, of the influence that industry, unions, all lobbyists and money has on politics in Australia," he said.

"But having any regulation needs to be effective regulation, rather than just make people feel good."

While the Greens have backed extending the remit of the lobbying register, neither Labor nor Coalition governments have tried to review the current system since it was created, leaving it up to piecemeal changes, the most significant of which was Tony Abbott's move to prevent political party executives from being registered.

Several third-party lobbyists who spoke to Fairfax Media, including WPP AUNZ's Justin Di Lollo and Executive Counsel Australia's Jannette Cotterell, have backed extending the register's reach.

But one of the biggest industry associations, the Australian Chamber of Commerce and Industry, remains opposed, on the grounds it was obvious who the chamber was working for.

Chief executive James Pearson said when politicians meet with chamber staff, "they know that we are speaking on behalf of our members" and the chamber "sees no need to alter the arrangements".

While Senator Xenophon agreed with Mr Pearson on extending the register's coverage, he said there was a need to also "ask the question of whether a record of meetings ought to be included on the register".

Professor Warhurst similarly said that "if it has been shown to work in Queensland, it is something I'd support".

But that approach, Mr Di Lollo said, could see the benefits of having a transparent register turn into "voyeurism".

He said while he supported a level playing field for all those seeking to influence government, the release of ministerial diaries was not needed.

"The reality is that something could be achieved with one phone call that in other circumstances could only be achieved in 10 meetings, so I think that could push it too far."

The government did not respond to requests for comment.