An Australian law firm is planning a $100 million class action against four companies it claims are underpaying people to spruik charities and products in shopping centres, on the streets and door-to-door.

Chamberlains Law Firm and the National Union of Workers (NUW) said the workers are employed as contractors by the companies and are paid as little as $3 an hour.

"These are people in Bourke Street mall and Rundle mall," the NUW's Godfrey Moase said.

"It is not right that someone gets paid just $3 to $4 an hour. That is terrible.

"They end up in a job where they are working 60 to 70 hours a week and pulling down just a few hundred bucks."

The minimum wage is $18 an hour, and the union will argue the companies breached the Fair Work Act.

"The level of non-compliance is so egregious," Chamberlains Law Firm's Rory Markham said.

"We have over 5,000 Australians, that on all the data we have seen are earning less than $5 an hour. Australians from your suburbs, your malls and knocking on your doors. How can that be acceptable?"

The companies to be targeted in the lawsuit are Aida Sales and Marketing, Credico Australia, Global Interactive Group (GIG) and PCA Group, whose clients have included Foxtel, Optus, Telstra, AGL, Alinta energy and the Red Cross.

In a statement, Red Cross CEO Judy Slatyer said the organisation was concerned by the allegations raised by the union and said their contracts require their employees to be paid at least minimum wage.

"We are deeply sorry to hear these stories and believe nobody should be subjected to such treatment," Ms Slatyer said.

"We value the people who fundraise for us … we have always insisted on legal and appropriate labour practices, including for contractors.

"Last year we also began seeking assurances from third party fundraisers and then, earlier this year, negotiated even stronger protections into contracts — including the move to a wage-based model in July."

The union said not every face-to-face or door-to-door salesperson for these companies had been underpaid.

But it said because many work on a commission, it can result in very low pay and it estimates 4,000 people are working in the sector in any given day.

Many people in the industry are young, but older Australians have also been involved in the sector.

The ABC has contacted the four marketing companies named in the complaint.

In a statement, GIG said they were shocked by the class action and would "strongly repudiate" the claims.

"It appears as though the information that Chamberlains and NUW are using is out of date and not reflective of our workforce engagement model," a spokesperson for the company said.

Long days, low pay

Two of those involved in the class action are Tim Smart and Clint Webb, who were door to door sales people for Aida, selling Foxtel and Austar products.

They were called "roadrunners" in the company, and were assigned territories to cover.

Mr Smart said he worked six days a week for about eight years, and averaged $400 a week.

"We were constantly on the road, living at pubs, caravan parks and it was unpredictable," he said.

"It was horrible the constant pressure to make sales, it was just soul destroying."

He said the low pay was one issue, but another was the intense pressure put on him and others to make sales.

"For the years that I gave them I don't think I'm ever going to catch up in life," he said.

"There are so many people in the same situation, not getting their superannuation, not getting their hourly rate, in an unstable situation, it is not right to treat people like this."

Mr Webb he said he worked 12-hour-long days to hit targets, six days a week, and his years in the industry has impacted him in unforeseen ways.

"It has affected me, and I just don't have trust in me, it has affected my family life. Relationships are just hard to form and to keep," he said.

"I'm not here about money, all I care about is other people in the same situation to give them what can happen."

The union said that to be part of the class action, people must join their union and pay union fees.

Chamberlain Law Firm said it will launch legal proceedings before the end of the year.