Queensland Investment Corporation (QIC) will take over Queensland Motorways from the state government in a transaction that would double the fund manager's portfolio of unlisted infrastructure assets.

Although a final price is still to be determined, QIC would have to pay market value, which is estimated at around $3 billion.

Queensland Motorways will be held as an asset under the Defined Benefit Superannuation Fund for public sector employees.

"QIC Global Infrastructure has long had an interest in Queensland Motorways Limited as it holds two mature tollroads in the fast-growing south-east Queensland region," QIC chief executive Doug McTaggart said.

"A further significant attraction is the CPI [consumer price index] linked revenue streams, which are well suited to the long-term liabilities of the Defined Benefit Superannuation Fund," he said.

The acquisition is timely, as many of Australia's superannuation funds have been eyeing unlisted infrastructure investments since the global financial crisis in an effort to become less dependent on equity returns.

McTaggart said he expected the acquisition process to be completed by 30 June 2011.

QIC Global Infrastructure's $3 billion investment portfolio currently comprises 14 unlisted infrastructure assets, including investments in Brisbane Airport, the Westlink M7 tollroad in Sydney, Thames Water, global ports developer and operator Grup Maritim TCB, and the Port of Brisbane.

On Monday, the state government raised $4.6 billion with the float of QR National, Australia's largest public listing so far.