New analysis from pro-transparency group Australian Democracy Network (ADN) shows the difficulties of keeping track of lobbying efforts from many corporations.
Businesses including big four banks NAB and Commonwealth, gas giant Santos and Qantas don't feature on the register - because they don't have to.
It's only third-party lobbying firms, working on behalf of many businesses, which need to sign up to the register and a code of conduct.
ADN believes more than half of Australia's biggest 100 publicly listed companies, and 70 per cent of the top 300 businesses - representing $1.6 trillion of value - are nowhere to be seen on federal lobbying lists.
Saffron Zomer, ADN's chief executive, said that meant Australians had little awareness how these businesses engaged with government and on what issues.
"Lobbying is part of democracy," Ms Zomer said.
"The problem is when corporate money buys a level of access and influence ordinary Australians could never afford."
Gambling groups Tabcorp and Aristocrat Leisure, media conglomerate News Corp and gas pipeliners APA Group are also among those without representation on the register.
The register lists which individuals have been contracted for each organisation, with some well-known names among 710 currently-listed lobbyists.
Former cabinet ministers Julie Bishop, Lindsay Tanner, Stephen Conroy and Christopher Pyne are among 22 former ministerial office-holders on the list, along with another 21 former politicians.
There are also three ex-premiers registered - Morris Iemma, Robert Borbidge and Paul Lennon.
Ms Bishop, who served as foreign minister for five years, has one client: Perth-based Twinza Oil, which is pursuing an offshore gas project in Papua New Guinea.
Mr Pyne, on the other hand, has built up a mighty customer base since founding his own firm in 2019, with 67 companies under his wing.
Several companies fall under portfolios Mr Pyne held during his time as a minister, including Bupa, Flinders University, and a range of defence entities.
The biggest lobbying firms are DPG Advisory Solutions - founded by former Liberal staffer David Gazard - with 104 clients, closely followed by TG Public Affairs, chaired by Mr Conroy, with 103 customers.
While the register is helpful for both politicians, to know who they are dealing with and any conflicts of interest that might arise, and the public, who are given some transparency, ADN senior campaigner Christian Slattery says it doesn't go far enough.
Both in-house and third-party lobbyists should be covered, Mr Slattery says, along with new regulations including timed bans for public officials becoming lobbyists, given their access to government information.
ADN believes ministerial diary disclosure should be enacted to make public meetings with lobbyists as close to real time as practicable.
That's unlikely under the Anthony Albanese-led government, which has pushed back against this reform during its time in office.
However, one transparency reform was enacted in July, with parliament making access-all-areas passes tougher to get, following advocacy by independent ACT senator David Pocock.Â
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