The Queensland mining union has defended more than $700,000 in credit card spending for the past financial year as entirely for "official business".

However, the union did not respond to requests from The Australian Financial Review to explain the scale of the spending or its almost 70 per cent jump in travel expenses over the past year, up by more than $300,000.

The Construction, Forestry, Mining and Energy Union Queensland branch, which has just 6369 members, was responding to reports of its auditor's formal concerns about the lack of evidence to back up credit card spending in the branch's 2015-16 financial statement.

CFMEU state secretary Tim Whyte said the reports were "misleading" and suggested the auditor's concerns related to only a small portion of the $721,116 in credit card spending.

"For a tiny fraction of that amount, a total of eight receipts were misplaced and the union has contacted the relevant individuals to ensure those expenses are accounted for," Mr Whyte said.

In a "qualified" approval of the union's accounts in December last year, auditor SRJ Walker Wayland said it was "unable to obtain sufficient appropriate evidence to verify that the credit card expenses ($721,116) that have been included as business expenses in the union's annual financial accounts have been incurred solely for business purposes".

CFMEU mining division secretary Andrew Vickers said on Monday that, in addition to the branch's 10 executives, 40 union delegates at mine "lodges" had access to union credit cards for travel purposes.

The union's financial statement recorded that, in 2015-16, spending on airfares, accommodation and car hire jumped from $455,274 to $765,285.

Mr Whyte said in his statement that "all expenses incurred by employees of the [Queensland branch] in 2015-16 were for official business".

"The CFMEU has reiterated with its staff the importance of keeping receipts for auditing purposes," he said.

The union's rising spending comes as branch membership revenue has been falling by millions of dollars a year, declining by 20 per cent since 2014.

The union also suffered a $3 million dollar downgrade in the value of its assets in relation to a related entity.

The auditor raised concerns over a $8.36 million valuation in 2015 of the property holdings of the CFMEU Industrial Union of Employees, Queensland Mining, Energy and Ports District.

It wrote down the value to $4.78 million and raised additional concerns about the lack of evidence to verify the entity's opening reserve balances.