A pledge to sell off the port of Fremantle this year by West Australian Treasurer Mike Nahan has been labelled "desperate" by the State Opposition.

Despite Premier Colin Barnett's previous prediction the proposed sale would not proceed this year, Dr Nahan this week said the port could be sold by July or August.

Dr Nahan made the statement on Monday, just hours after Moody's ratings agency downgraded WA's credit rating to double-A two.

Shadow treasurer Ben Wyatt said accelerating the timeframe for selling the port smacked of a fire sale.

"When a desperate government resorts to desperate measures, you get a bad outcome, not just in terms of spending decisions but on the value of property that the government is proposing to sell," he said.

"It smacks of a government desperate for financial credibility, giving the taxpayers an awful outcome."

The port is one of a string of public assets being sold by the Government as it attempts to contain and pay down spiralling debt.

The Government plans to sell a long-term lease for the facility.

Estimates on the value of the transaction have ranged from $6 billion down to $2 billion.

Speaking on Monday, the Treasurer was determined to push the sale through early in the next financial year.

"There have been people including the Premier who voiced concern about the timing of it — for getting it through Parliament," Dr Nahan said.

"But I am confident, and I'm in charge of this, that the sale of the Port of Fremantle is going on target.

"And it is the time to sell it. The demand is significant, the prices being offered are large, in fact historic. I'm very confident the port of Fremantle will get across the line on schedule and with substantial earnings."

Sale could delay outer harbour: WAFarmers

However the State Opposition is not the only group concerned by the apparent urgency of the planned sale.

WAFarmers president Dale Park said it was hard to escape the sense it was being rushed.

He also remained concerned about the impact of the sale on a future outer harbour at Kwinana.

"If the sale goes badly, and if it is rushed … we might not get an outer harbour for another 50 or 100 years," Mr Park said.

He said the concern was shared by other users of Fremantle Port, who were worried that a high sale price might leave the new port operator with insufficient cashflow to build an outer harbour.

"Don't rush it. There is no hurry.

"I think the potential to rush a decision could have ramifications for this state a long way into the future."

Mr Barnett has previously said the construction timeline for an outer harbour should be determined before the port was privatised, to avoid the prospect of later compensation.

In a statement, Dr Nahan said the State Government was reviewing a detailed scoping study report, and would consider all aspects of the sale before making a decision to proceed with the transaction.

"The proposed divestment is on track with the sale expected in the second half of 2016, although the timing will ultimately be determined by the legislation's swift passage through Parliament," he said.

Dr Nahan said he was pleased with the "great deal of interest" being shown in securing the long-term lease of Fremantle Port.