The Cavenagh Street “shade” structure is expected to cost taxpayers more than $100,000 to maintain this financial year – up from $13,400 last financial year – government figures obtained by the Opposition show, with Infrastructure Minister Eva Lawler ruling out replacing the vines that have still not covered the structure in four years.

The new figures also show the structure cost about $698,285 more to build that the government has previously claimed. At the time of its completion in 2018, the government said it cost $2.7 million but now say the total cost is $3,398,285 – about 20 per cent more.

CLP Infrastructure spokesman Gerard Maley said the figures released through written questions – which the government are obligated to answer – show an anticipated $100,000 to maintain the vines per year for the next 12 to 24 months, but also claimed the anticipated cost for this financial year was “in the order of $90,000”.

The responses gave no explanation for the discrepancy, but showed the cost for the first quarter of this financial year was $48,303 excluding GST, which if extrapolated over the full year would be $193,212.

The figures show that the cost last financial year was $13,370.

The Minister’s response indicated the money included the vine maintenance and pruning, fertilising, and irrigation repairs as required.

“Not only has this been a huge waste of taxpayer money from the get go, but the vine itself has been a complete failure and it’s taken the Minister since May 2022 to decide on an alternative vine to plant instead, yet there’s still no clear answer on what this government is going to do,” Mr Maley said.

“How maintenance of this vine has increased six-fold since 2021 is laughable, when the vine is lucky to have reached the top since its planting.”

In her response, Ms Lawler said there were no plans to replace the existing vine, but they had been undertaking trials to plant other species to “coexist with” the current vine.

An ABC report from February last year said documents obtained under Freedom of Information laws found the structure had cooled temperatures underneath it by less than 1 degree Celsius in the two years after it was built.

The documents said data from mid-2020 showed the structure reduced average peak heat by 2 per cent, from 32.1C to 31.4C but researchers said the structure’s cooling performance would improve once the vines are fully grown, which has still not occurred.

Changing projections for when the shade structure would provide shade

On the last day of October 2018, then-Infrastructure Minister Nicole Manison said the Rangoon creeper and orange trumpet vines would grow to full size in 12 to 18 months. The following February, Ms Lawler said it was anticipated vines should have covered the structure by May 2020.

In September 2020, the NT Government said in a statement to media the original forecast period for the vines to reach a “reasonable level of coverage” was 18 months, which contradicted the press release Ms Lawler put out in early 2019.

They said the vines had suffered from an infestation in their infancy as well as overzealous pruning which had contributed to sluggish growth.

The 55-metre structure on Cavenagh Street was completed in late 2018 as part of the NT Government’s plan to transform Darwin into “a cool, tropical, world-renowned destination”, after a 2017 heat study revealed some surfaces in the city were hitting temperatures between 45C and 67C.

In December 2020, Tony Cox, the director of Landscape architects Clouston Associates, the firm contracted by the Department of Infrastructure, Planning and Logistics for the plantings, told ABC Darwin radio it was extremely difficult to determine just how quickly the vines would grow. He questioned the government’s initial prediction of having full cover in 18 months.

“We are talking about a growing plant, and it’s very difficult to determine just how quickly they will grow and, and so forth,” he said.

“Then, the other one too that we need to consider here is, the whole shade structure itself is something like over 1,000 square meters.

“It’s a very large structure, and to even get a tree to get a coverage on such a large area would take five trees and probably 15 years, so the whole project was experimental to some extent. The vine itself … has done well. But we are asking it to grow essentially like a tree, not just a vine. It has to behave like a tree and cover a very, very large area.”

But former NT Parks superintendent John Antella told the ABC from his observations the orange trumpet seemed to have died.

“I could be wrong, but that’s just what I’ve noticed driving by,” he said.

“…I wouldn’t pull it out. I think you’ve got to live with it. But you know, you’ve probably got to wait 18 years for it to do what it was supposed to do in 18 months.

“So they probably need to change the sign down there [on Cavenagh St], to you know, say approximately 18 years [from 18 months]. I am not being critical of anyone who was involved, but the reality is the Rangoon creeper has been historically known as a rambling shrub and a clambering shrub, it’s not described as a climber.

“If they wanted a shade structure there, they should have really put an impervious cover over the top of the shade structure, which was heat reflective, and we would have had, you know, a shade structure, which we’re not going to get now.”

This would mean a child who was born when the vines were planted would have finished high school by the time they had covered the structure.