Almost a third of the nation's top 200 firms pay an effective tax rate of 10 per cent or less, the union leading the fight against corporate tax avoidance says.

Hearings began today that are part of a Senate inquiry into corporate tax avoidance and minimisation, with tech giants Apple, Microsoft and Google being grilled about the way they structure their finances.

United Voice, which represents workers in industries including cleaning and hospitality, published a study last year into how much tax companies pay relative to their total profits, prior to a range of deductions.

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"Nearly a third of the ASX 200 companies have an effective tax rate of 10 per cent or less, and that contributes to a loss of up to $8.4 billion in revenue, and that's just the Australian top companies," the union's national secretary David O'Byrne told the inquiry.

"A report last week showed that the top 900 companies pay, on average, an effective tax rate of around 19 per cent, less than the average Australian worker."

One of the senators in the inquiry cited a senior Treasury official giving evidence at a previous Senate hearing who dismissed the $8.4 billion loss from "tax planning" as "patently false".

However, Mr O'Byrne stood by the report, and said the debate over what the true losses were reflects the paucity of information available to the public about how much tax large companies are paying.

"If there is this level of disagreement about the stats surely that indicates that that's a problem, surely that indicates there is a lack of public information which is easily accessible, which is easily explained to people to justify the actions of certain corporations," he responded.

Mr O'Byrne argued that there are simple ways the Federal Government can tackle the problem.

"Compulsory country-by-country reporting for all Australian-listed companies, disclosure of all subsidiaries including an explanation of the purpose of those located in tax havens and also, importantly, ensuring that the Australian Tax Office is adequately funded and staffed to do their work," he added in a not-too-subtle barb directed at Coalition senators, after the Government laid off around 3,000 Tax Office staff in 2014.

Tax office says 100 issues in dispute with 69 large firms

However, the Australian tax commissioner, Chris Jordan, used an interview with ABC's The Business program to defend the Australian Tax Office's performance, saying it is already investigating many major companies.

"There are 69 companies that have a turnover of more than $5 billion a year, but they represent 42 per cent of the total tax take," he told presenter Ticky Fullerton.

"We have a real-time presence, a continual presence with them and we know exactly what they're doing - we might not agree with everything they're doing because we have around 100 issues that are in dispute."

Sorry, this video has expired Interview with tax commissioner Chris Jordan ( Ticky Fullertons )

Mr Jordan revealed that a large number of those disputes are with technology firms.

"What we're trying to say here is, 'you have commercial activity in Australia - clearly - so it just seems to be too disconnected to reality to put forward the assertion that there's no taxing rights here'," he explained.

"We have 12 significant, deep audits going on right now with 12 major tech companies, challenging what they've been putting to us over the years under existing laws."

When asked about reports in the Fairfax press that BHP Billiton and Rio Tinto saved $750 million a year in tax by using Singapore as marketing hub to divert profits to that lower-taxing jurisdiction, Mr Jordan refused to comment on specific companies and investigations.

"It probably puts BHP and Rio up in that top 69 companies, clearly. They have turnover of more than $5 billion and we are disputing marketing hubs of large companies," he responded

"I can't talk about individual companies, all I can say is marketing hubs are a huge concern for us. They are growing. It's not just in the mining area; it's being used in a much wider sense.

"Often the people in these hubs are the most profitable people in the organisation. So we are challenging them."