· In short: The cost of port infrastructure for the new Spirit of Tasmania ferries has ballooned, from $90 million to $375 million, after a scramble to secure a new contractor.

· Along with the port cost blow out, the new ships are already late — with one estimate being that for every 12 months of delay, the Tasmanian economy takes a hit of $350 million.

· What's next? The first of the new ferries is expected to begin its journey from Finland to Tasmania in the third quarter of this year.

The cost of upgrading the port that will house the new Spirit of Tasmania vessels has blown out to more than four times the original estimate and won't be ready until after the ships' arrival, the government has revealed.

Tasmania's ferry fleet is more than 25 years old with the two replacement ships already overdue. The existing facilities in Devonport were due to be upgraded to accommodate the new ships at a cost of $90 million, according to a quote from 2020.

However, a new quote for the completion of the works has come in at $375 million, more than four times the original estimate.

In 2017, the then Hodgman government promised to deliver "two new, purpose-built Spirits of Tasmania vessels in 2021, two years ahead of schedule".

When complete, the ships will increase passenger capacity from 1,400 to 1,800, bring the number of cabins from 222 to 301, and allow for 40 per cent more passenger and freight vehicles.

Three years post due date and those Spirits are still halfway across the world — with the first one completing a successful sea trial in the open seas off Finland on Tuesday.

The Tasmanian government said it now expects the first of the new ships will be ready for "delivery" in the third quarter of 2024, with the second due in the first quarter of 2025.

Economist Saul Eslake previously estimated every 12 months of delays would cost the Tasmanian economy $350 million.

But it's not the only cost associated with the project.

It was revealed by Labor during question time that TT-Line — the operator of the Spirit of Tasmania ferry service — had bailed out Finnish shipbuilder RMC to the tune of $81 million, two days before the state election.

And then there are the infrastructure issues.

The new Spirits are to be berthed at berth 3 in Devonport in Tasmania's north west, but it requires some significant upgrades. They are also delayed.

Infrastructure Minister Michael Ferguson said TT-Line dropped its preferred tenderer in the middle of an election when the government was in caretaker mode.

"The decision to rescind the preferred-contractor status was a unilateral decision taken by TT-Line within its responsibilities due to cost escalations," Mr Ferguson said.

"The government's position remains that appropriate berthing must be in place at Devonport to support the arrival of our new ships.

"This expectation extends to TT-Line ensuring that appropriate fuelling arrangements are in place to support the new vessels both at berth 1 and berth 3."

The government said the increase, from $90 million to $375 million, was due to global increases in materials and construction costs.

Not only will it cost more, but the berth won't be ready on time, with the new estimated completion date January 2026.

Just last month, Premier Jeremy Rockliff told parliament the berths needed to be ready for the arrival of the ships.

"It is my expectation that all best efforts will be made to ensure there is appropriate infrastructure in place at East Devonport to support the arrival of the new ships," he said.

Now the government has had to ask TasPorts to undertake "additional and immediate work" to upgrade berths 1 and 2, so they can act as temporary homes for the arrival of the first new ship.

It is not yet known how much these upgrades will cost.

Mr Ferguson also said the government will "provide enhanced oversight" of both projects with TT-Line and TasPorts required to provide advice to shareholder ministers.

Labor leader Dean Winter argued Mr Ferguson should have already been doing that.

"What on earth has he been doing up until now?"

"As with so many major projects promised by the Liberals, the Spirits program has been plagued with delays, cost blowouts and secrecy – and it's taxpayers and the Tasmania economy that pay the price."

In a statement, TasPorts chief executive Anthony Donald said the section of berth 3 that TasPorts was responsible for was on track and on budget.

"We stand ready to work with port partners to ensure the Port of Devonport is equipped for the arrival of the first of the new Spirit vessels," said Mr Donald.

"In addition to our current works, the Tasmanian government has asked us to assist by undertaking an upgrade of berths 1 and 2 to enable the arrival of the vessel, and we'll work closely with TT-Line to do that.

"Work will progress immediately starting with discussions with engineering consultancies and suppliers, infrastructure and construction contractors and other port partners."