The union also incorrectly reported $2.9 million worth of donations in 2015, as proceeds of the union’s investment in the IQ Apartments, rather than a donation from the club, as was actually the case.

But union secretary Jason O'Mara, who was not involved in the reporting at the time but has since investigated the issue, said the errors in reporting came about due to differing interpretations of the union's accounting advice from one year to the next.

The Canberra Times has previously revealed the Tradies Club gave the union $3.8 million in proceeds from the controversial Dickson land swap deal with the ACT government in 2014-15.

The land swap involved the club buying a Dickson carpark from the government, in exchange selling government two of its own blocks nearby, in a deal the ACT Auditor-General found gave the club up to $2.65 million in concessions.

That deal is now being investigated by a Legislative Assembly committee inquiry.

Griffith University’s head of accounting, Dr Reza Monem, reviewed the union’s financial reports from 2014 to end of March 2017 for The Canberra Times, finding some key financial reporting errors.

The reports show the union operated at a $36,178 loss at the end of 2013, and without the extra funds from the club, it would have been unlikely to cover more than $1.5 million in legal costs it incurred from 2015 to the end of March 2017 without going into further debt.

Those legal costs are also expected to again rise in coming months, as the union and Mr O'Mara fight allegations by the Australian Competition and Consumer Commission legal and the Australian Federal Police they engaged in alleged cartel behaviour over steel-fixing and scaffolding contracts.

But most concerning to Dr Monem was the mis-reporting of $2.9 million the club gave the union in 2015, saying it beggared belief the CFMEU did not know the source of the funds at the time.

“It is a question of general business acumen, imagine your total revenue is $4.5 million and out of that, you’ve received $2.9 million from a single source, of course you know the source of the donation.

“You can’t simply misclassify it,” he said.

“The 2015 report shows a major misclassification error, a donation from Canberra Trademen's Union Club to the CFMEU was reported as investment income and then in 2016 they corrected it.

“But my question is: how could they not know what is the source of this $2.9 million?”

Mr O'Mara said the issue was fully described in the union's financial accounts, claims of such errors were attempts to sensationalise the matter, and any suggestion of a lack of transparency was totally wrong and offensive.

"Obviously, the union knew the source of funds," he said.

"At the time of the 2015 report, on our accountant’s advice, we understood them to have been correctly described.

"When we were subsequently provided with advice that our categorisation was incorrect we acted upon that advice which resulted in the re-categorisation which has been described in our financial reports."

The reports show the union received $575,322 in donations in the 2014 calendar year, and $374,315 in 2015, while a further $2.9 million was also reported in 2015 as an investment distribution from the union’s investment in the Marque on Northbourne apartment development.

But that $2.9 million figure was then corrected in the 2016 financial report to show it was actually a donation, with the union receiving $3,873,463 in total donations in 2014 and 2015.

The club’s financial records show it gave the union $3.8 million in payments during the 2014-15 financial year, making it the largest annual contribution the club gave the union from 2010-11 to 2015-16.

Dr Monem said the mis-reporting of the $2.9 million figure in 2015 was not like forgetting the source of $5 in revenue, and given it was the biggest single revenue line item in the CFMEU's books that year, it raised serious questions about the union’s financial transparency.

While the reports do not detail the specific legal cases the union’s costs related to, the union faced the Royal Commission throughout 2015 and was still battling the resulting court cases through 2016 and 2017.

The documents show the ACT union’s legal costs rose from $35,878 in 2014 to $295,446 in 2015, then soaring to $1,023,127 in 2016 before falling back to $327,696 in the first three months of 2017.

Of those total legal costs, litigation cost the union nothing in 2014, rising to $169,602 in 2015; $775,209 in 2016 and $277,986 in the first three months of 2017.

Mr O'Mara said the given the current political climate, and due to relentless attacks on unions by the [federal] government, and its 'attack dogs', like the Australian Building and Construction Commission [ABCC], it was unfortunate that from time to time legal costs are incurred.

"We have disclosed our legal expenses in accordance with the relevant Fair Work Commission reporting standards," he said.

"We note that in 2017, we obtained information from the ABCC by Freedom of Information, which indicated that [it] spent $1.7 million in legal costs for two prosecutions of the ACT Branch run in the period 2015 to 2017."

The club underwrites the union as a going concern and footnotes to the reports showing the union is economically dependent on the club detail some of the financial support the club has given the union.

But those notes, in each of the financial statements reviewed, do not detail the full extent of the $3.8 million the club gave the union, and Dr Monem said he could not comment on why that was the case.

The reports also show the union returned $1 million back to the club via a secured loan, which the club had paid back to the union, with five per cent interest, by the end of March 2017.