A consortium led by Australian investment bank Macquarie on Wednesday made a surprise US$5.5 billion bid for Tatts Group, throwing into question a planned merger between the gambling giant and rival Tabcorp.

Tatts said it was assessing the unsolicited offer from the consortium -- which includes First State Superannuation Scheme, North Haven Infrastructure Partners and  Kohlberg Kravis Roberts & Co -- although its directors were still supportive of the Tabcorp merger.

Tabcorp and Tatts announced in October they were combining forces to create a  powerhouse taking bets on horse racing and sports events across the country.

"Tatts has not yet formed a view on how the indicative proposal (from Macquarie) compares to the proposed Tabcorp merger," it said in a statement.

"The Tatts board and its advisers will assess the indicative proposal including its terms, underlying financial assumptions and conditions, and will provide a further update on the outcome of that review as soon as practicable."

It added that "the directors of Tatts continue to believe the proposed Tabcorp merger is in the best interests of Tatts shareholders and unanimously recommend the proposed Tabcorp merger, in the absence of a superior proposal and subject to an independent expert concluding the proposed Tabcorp merger is in the best interests of Tatts shareholders".

Shares in Tatts closed 8.45 percent higher at Aus$4.49 on Wednesday.

Under the Aus$7.3 billion proposal, Tatts is valued at between Aus$4.40 and Aus$5.00 per share. The Tabcorp merger valued Tatts at Aus$4.34 per share.

No comment was immediately available from Macquarie.

Tatts Group has a betting shop network in the states of Queensland, South Australia and Tasmania and also operates a lotteries business. 

Tabcorp Holdings runs similar operations in Victoria and New South Wales, and has a broadcasting and media arm built around Sky Racing.

Both companies also compete in online gambling.