An assessment by Standard & Poors prepared for investors considering buying the Port of Melbourne has concluded the likelihood of a rival port being built - either at Hastings or Bay West near Werribee - is remote.

It found both proposals for a second port were more than 50 kilometres from the city, making the cost, including for the extra road and rail infrastructure needed, hard to justify.

It also concluded the Port of Melbourne had room to expand, with the current capacity set to almost double to handle about 4.5 million containers.

"Further, the capacity could increase in excess of 7 million (containers) through limited investment that would be insignificant compared with the capital required for a new port. Practically, therefore, a second major port is unlikely to happen anytime soon."

Political debate over the need for a second port has been simmering for years. The former Napthine government was adamant about the need for a deep water port at Hastings.

It set aside $110 million in April 2013 to begin planning for the expansion of the Port of Hastings, claiming it would eventually handle up to 9 million shipping containers a year.

In Opposition, Labor's official position was that it would refer the question of the best location to Infrastructure Victoria, although in the lead up to the 2014 election it talked up prospects for a port on the other side of the bay near Werribee, given the proximity to Geelong and freight and logistics facilities in the west.

Since the election, Labor has fallen largely silent on the issue of the best location. Earlier this month Special Minister of State Gavin Jennings formally instructed Infrastructure Victoria to examine when Victoria will need a second port and where it should be located.

After a long negotiation, the Andrews government passed legislation to privatise the port earlier this year, with hopes the sale will raise at least $6 billion for Labor's level crossing removal program.

But the Standard & Poors assessment suggests the actually price could be even higher, finding its business model would be underpinned by an "entrenched and dominant" market position.

"The Port of Melbourne is at the centre of the Victorian logistics chain and in proximity to a large portion of the state's population, which cannot be replicated by any potential competing port."

A spokeswoman for Ports Minister Luke Donnellan said the timing of the second port would be "demand-driven", timed to coincide with the point at which the Port of Melbourne reaches capacity.

"Consistent with the Victorian Government's election commitment, Infrastructure Victoria will provide advice regarding a location for Victoria's second container port," she said.