Only days before treasurer Josh Frydenberg is set to table the legislation, Australia's media firms have proposed additions to the bargaining code on revenue sharing.

In a joint statement published today, major publishers and TV companies have listed "essential changes" they say must be implemented to ensure a level playing field with the digital platforms.

Major metro newspapers and their digital sites published the open letter today, meeting concerns that Google and Facebook had been active in last-minute lobbying to modify the proposed legislation.

Signatories include News Corp's Michael Miller, James Warburton of Seven West Media, Nine chief executive Hugh Marks, Guardian Australia chief Dan Stinton, and Free TV chief executive Bridget Fair.

It says the News Media Bargaining Code "must include"

-final offer arbitration, limiting incentives to make ambit claims;

-strong protection against discrimination: the letter says Google has used its bargaining power in other jurisdictions to avoid making "a fair contribution" towards the cost of content creation, and Facebook "has threatened to do the same in Australia";

-coverage of all services including the the full suite of Google and Facebook products, including Instagram; and

-information exchange to ensure a fair and balanced commercial negotiation.

The letter says signatories support Google's public commitment to helping fund the future of Australian media. "That is certainly something that we welcome," it says.

"Supporting a fair and reasonable code is the first step; the code is essential to arrest further declines in professional news content in Australia - something our democracy depends on."