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.