Woolworths is facing a class-action lawsuit over a 2015 share price plunge sparked by a shock profit downgrade.

Law firm Maurice Blackburn, which has opened an online registration portal for aggrieved shareholders to sign up, says the claim could well exceed $100 million, according to news.com.au.

The action alleges the supermarket breached the Corporations Act by failing to keep investors adequately informed of "significant risks" to its profit projections.

Andrew Watson, class actions principal at Maurice Blackburn, said that while investigations were continuing, it was clear Woolworths had known it was significantly behind on its profit projections as early as October 2014 but continued to maintain its profit guidance until the publication of its half year accounts in February 2015.

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In August 2014, Woolworths provided guidance that its FY15 full-year profit would increase by between 4-7 per cent.

When the profit guidance was revised down on February 27, Woolworths shares plunged, losing $4.66 or 13.7 per cent of their value in two days.

"While there is a clear path to meet the guidance provided of net profit after tax of four to seven per cent in FY15, we have decided to provide ourselves with additional flexibility to make the necessary investments to deliver on our long term plans and the associated shareholder value creation," then chief executive Grant O'Brien said at the time.

In a statement, Watson said that when corporations failed to "abide by the laws requiring they make timely and accurate market disclosures, these aren't mere technical breaches".

"It causes loss to shareholders, undermines the integrity of the market and distorts the efficient allocation of capital that could go to more deserving companies," he said.

"The end result is that shareholders, both individual everyday Australians and large institutional investors entrusted with members' savings such as large superannuation funds, unwittingly suffer the consequences and lose out in a major way."

Wayne Attrill, senior investment manager at global litigation fund IMF Bentham, which is supporting the class action, said the case would only proceed if enough shareholders signed up.

"This is a chance for investors who believe they were deprived of information on the true state of affairs of the company standing up and being able to access a meaningful redress mechanism while sending a strong message to the company that such breaches aren't acceptable," Attrill said.

"A strong culture of good corporate conduct is as important as ever when it comes to attracting future investment in our economy, and strong enforcement mechanisms through the public regulator and private redress via class actions help reinforce that message."

Woolworths has been contacted for comment.