The project has also been delayed after excavators had to refill more than half the area with sufficiently strong material.

An update on the project will be considered at Monday’s meeting of Armidale Dumaresq Council.

Nightwork begun on April 23 was delayed when workmen building the runway’s link section struck a soft underground which had to be replaced with 300 millimetres of rock ballast, public infrastructure director David Steller said in his report to Council.

That was stabilised with cement to provide a firm foundation for the taxiway’s pavement.

“This work has added significant cost to the project and caused contractual delay claims that are payable under the contract,” Mr Steller said.

Council has borrowed $3 million for the airport’s upgrade to be funded by airport charges and another $3.2 million for Dumaresq Dam’s upgrade, to be funded by water fund user charges.

It will try offset some of this spend by applying for Local Government Infrastructure Renewal funding if it becomes available.

The cost blowout for the airport upgrade includes additional drainage works, ($51,770) extra earthworks ($417,000) concrete encasement and drainage ($50,400) and additional runway lighting ($25,100).

Councillors are tipped to debate the costs on Monday.