According to Queensland Treasury Corporation's latest financial analysis of the council's finances, the council's gross debt more than doubled over the past five years to $2.61 billion.

"BCC's large capital expenditure program has resulted in gross debt increasing by $1.47 billion over the past five years to $2.61 billion in the 2014 financial year," the QTC report says.

The council's operating deficit also worsened to $80 million, down from a predicted budget operating surplus of $12 million.

However the report points out that one-off budget items such as the decision to lease the Legacy Way tunnel pushed the operating deficit into debt by a further $68 million, while a re-classification of a $66 million Queensland Reconstruction Authority grant worsened the operating debt for 2013-14.

Brisbane City Council also "wrote down" $1.24 billion for the value of its $1.5 billion Legacy Way tunnel – still being built - and its Go Between Bridge in 2013-14.

Both these pieces of transport infrastructure were leased to Queensland Motorways, which was on-sold to a consortium led by New South Wales toll road operator Transurban.

The report points out there is still no signed agreement for sewerage utility Queensland Urban Utilities – which evolved from Brisbane City Council in 2012 – to pay back the $1.24 billion loan it received from the council.

Queensland Urban Utilities is in the process of replacing its loan from the council by borrowing directly from QTC.

The $1.24 billion should be paid back over three years.

However, as the QTC report says: "While repayment of this loan is included in BCC 2015 financial year budget and forecasts, QTC understands that QUU has not yet fully considered or agreed to the transaction."

A QUU spokeswoman said the new loan with QTC was still being finalised, but provided no details.

"Queensland Urban Utilities is in discussions with its shareholding councils, Queensland Treasury and the Queensland Treasury Corporation in relation to refinancing its shareholder loans," she said.

"Discussions are progressing well and according to an agreed timeline."

Overall the QTC report says the climb in the council's debt has peaked, large loans are due to be repaid and gross debt should now decline.

"With the planned repayment of its debt as part of normal debt repayments, coupled with funds from the Australian Government for Legacy Way, the lease of tolling concessions and the potential repayment from Queensland Urban Utilities, BCC forecasts principal repayments of $601 million in FY 2015 and a further $510 million in FY2016," it says.

The report shows the council still owed $431 million for the Clem7 tunnel, $251 million for the Go Between Bridge and $896 million for the Legacy Way tunnel.

Lord Mayor Graham Quirk pointed out that the 2013-14 QTC report shows the council finances were for the third year rated as "strong", with a "neutral outlook".

"There are not too many councils around Queensland that are in that position," Cr Quirk said.

He said it was common for organisations to write down assets and warned critics their assumptions were inaccurate.

"The last time I looked the Clem7 was still there, the Go Between Bridge was seeing traffic move across it," he said.

"They are asset classes that are there for a long, long time."

Council's opposition leader Milton Dick questioned what the additional $142 million received in rates revenue - $769 million in 2012 and $911 million in 2014-15 - had been spent on.

Council's finance committee chair Julian Simmonds said four of the last five Labor budgets (before 2003-04) had operating deficits.

"The truth is this has come about because of one-off accountancy decisions made by the Queensland Audit Office," Cr Simmonds said.

"They have chosen to re-classify some things at the last minute. Up until then we were on track to receive a 3 per cent operating surplus."

Council's finance committee spokesman Kim Flesser said rates revenue had grown from $769 million in 2011 to $911 million in 2013-14.

"That is an 18.4 per cent increase in just four short years," he said.

QTC is the main source of borrowings for Queensland councils and in certain circumstances, the Queensland government.