FREMANTLE had room to grow for at least 25 years, according to Treasurer Mike Nahan.
Dr Nahan said a move to Kwinana before Fremantle was at full capacity could add costs to all port users and customers.
It was likely to be a lot of planning around the costing's, design and capacity of the existing ports would be.
However Dr Nahan said the outer harbour was likely to cost between $2 billion and $5b.
"We have about 740, 20ft equivalent containers going through the port, growing at 4 per cent a year," he said.
"We believe we have 25 years growing at that rate, before it reaches its maximum capacity, so the outer harbor is not needed immediately.
"It will take 10 years to build the outer harbor because of environmental process will be very rigorous."
Dr Nahan said there is a whole range of "nonsense" around the issue of timing of the outer harbor.
"If we build the outer harbor early, you will increase the cost of using the port immensely," he said.
"You have a port that is in the third of its capacity, if you build a new one, you have to pay for it."
Dr Nahan said at this stage the details around how to fund it and when to build it will need to worked out.
"So it would be 25 years before an outer harbour is fully needed and 10 years before we seriously need to build it," he said.
"But we need to get onto the design earlier, as it might not be able to be built.
Dr Nahan's concerns on construction of the outer harbor link to the process of environmental clearances, but he was adamant he would endeavour to seek approval.
"We just need to get to that point and see," he said.
"If it can't be built what will we do? But we will work on that."
Dr Nahan presented the details of the privatisation to the Chamber of Commerce and Industry as well as an industry forum in Northam last week.
He suggested that a move to Kwinana before Fremantle was at full capacity would be hard to fund and therefore the timing had had to be considered.
"If you build it prematurely, it would be hard to fund," he said.
"So when it reaches capacity the government of the day can approach get a plan they can build it.
"In the sale, we would identify an agreement to build the outer harbor, who has the capacity and when it is needed they will design, build and operate and for the public, and we will set aside a great deal of money."
He said therefor under the sale process, the winning bidder would get first right to develop the outer harbour.
He said this would occur at the State's discretion and the government of the day would be in consultation with the bidder.
If the winning bidder declined, Dr Nahan said the State could offer the project on similar terms to a third party.
"It will cost $5 billion to build the outer harbour and that's including the island port and the 1km long rail track, infrastructure and will take about 10 years to build," he said.
"We think we would get $2b for the port, we have already had a lot of interest in it, but are yet to discuss the details.
Mr Nahan said at this stage live exporters, lobby groups, CBH and other users of the port had indicated to them they were "on board".
"We are protecting CBH, we are ensuring access and pricing regime for anyone exporting and importing through