The proposed sale of the Australian Securities and Investments Commission branch, flagged under former Treasurer Joe Hockey, has entered the final stages with the deadline for private bids due on Monday.

Unions and tax campaigners have entered a last-minute appeal to stop the sale, which they say will impede efforts to reveal more about the beneficial ownership of companies in Australia – a crucial tool in the fight to stamp out crimes such as tax evasion and money laundering.

Finance Minister Mathias Cormann left the door open to keeping the databases on Thursday, telling Fairfax Media the government "has not made a decision on whether it will go ahead with the sale".

"The government will consider the final bids before making a decision on any further steps," he said.

The chairman of the Australian Competition and Consumer Commission, Rod Sims, who will oversee any purchase, said he had met the department of finance over appropriate regulation.

"It's a monopoly asset so we want to make sure that when a private company owns a monopoly asset … Obviously they will be charging as much as they can so you need some regulatory arrangements stopping them from taking advantage of that," he said.

Mr Sims said the government would need to introduce strict rules for any private buyer.

"What we've seen, with some ports for example, is that there has been privatisation without any real regulation," he said.

"So we just want to make sure that doesn't happen here."

The government says it will introduce legislation to protect access to corporate data but has not revealed what form this would take.

ASIC holds more than 10 million records of Australian companies, which include financial statements, director names, director backgrounds, and shareholder information.

It also managers the registers of company auditors, managed investment schemes and the more than 22,000 financial advisers recently placed on the Financial Advisers Register.

In a letter sent to Treasurer Scott Morrison this week, a group of more than 30 community groups including Oxfam, United Voice and World Vision, said a sale of the registry would jeopardise efforts to increase corporate transparency.

"We are writing to seek your assurance that the ASIC corporate register will not be privatised to become a private monopoly," the letter said.

"The Panama Papers have demonstrated the need for governments around the world to take strong action against shell companies with concealed ownership, which facilitate a range of serious crimes including tax evasion."

A GetUp! campaign has also called the proposal a bitter recipe for "rampant tax dodging and shady shell companies".

The Australian chairman of Transparency International, Anthony Whealy, QC, said selling the registry would undermine commitments by the Turnbull government to open up corporate data.

"The worry always is that once it passes out of public control into private hands we just do not know what restrictions will apply," he told Fairfax Media.

Mr Sims said protecting access to data was particularly important "if the company that buys it is vertically integrated – so that not only are they managing the register but they have a downstream business that wants to take advantage of the data."

So far speculation about potential buyers has circled around credit agency Veda, which was recently bought out by US firm Equifax, share registry group Computershare and Canada's Information Services Corporation.

The tender for the sale is closed to the public.