The Premier Colin Barnett has conceded he may have made a "slight overstatement" when he told Parliament a joint letter written by the chairmen of Verve and Synergy had called for the entities to be merged.

The Government has introduced legislation to Parliament to merge the two utilities, after announcing the move in April.

At the time, Mr Barnett talked up a letter he received from the chairmen of both utilities, saying it influenced his thinking on a merger.

"I guess one of the telling points came mid-last year when the chairmen of both Verve and Synergy wrote to me and said the two should be amalgamated," he said.

"And I took that advice," Mr Barnett told the media on April 10.

In Parliament yesterday, Mr Barnett repeated the statement.

"I'm just telling you that the chairmen of the two utilities wrote to me in a joint letter, saying: 'This does not work; put it together.' Fact," Mr Barnett said.

But under fire from the Opposition during Question Time today, Mr Barnett released the letter, and conceded it never specifically called for a merger.

"Mr Speaker, look, perhaps what I said yesterday might have been a slight overstatement," Mr Barnett said.

"Mr Speaker, [the letter is] not actually calling for an amalgamation, but certainly making comments along those lines."

In the March 2012 letter, which was tabled in Parliament, then-chairman of Verve Energy, David Eiszele, and the then-chairman of Synergy, Michael Smith, stated "the Government's key objectives for the proposed merger are not entirely clear to us".

It goes on to say: "The Corporations are keen to avoid being involved in what might turn out to be a less than satisfactory process or outcome, and to this end we suggest that the Corporations undertake a conventional due diligence exercise before a decision is made to proceed".

The Opposition's spokesman for Energy Bill Johnston has called for an inquiry into the legislation.

"What we now find out, in fact, is that the Government is doing exactly what they were told not to do," he told Parliament.

"No wonder the Premier has taken 18 months to table this letter, he would have died of embarrassment if it was tabled before the [legislation] debate."