Fronting a public hearing into the Hunter Valley Operations' plans to operate for another two decades, Newcastle-based Leah Stevens outlined the financial hardships borne by the region from fossil fuel-driven climate change.
Roughly a third of the Newcastle region is now designated as flood-prone land, she said.
Annual insurance quotes of up to $14,000 had been provided to her clients buying in vulnerable areas.
"I cannot give any guarantee to my clients that these costs aren't going to increase in the future," she told the NSW Independent Planning Commission hearing in Branxton.
"With our changing climate, I am sure that they will."
The Yancoal and Glencore joint-owned Hunter Valley Operations is one of NSW's biggest coalmines and extending its life will result in more greenhouse gases released on site as well as abroad when coal is burned.
The project also employs 1500 people and supporters see the extension as a key source of jobs and economic stimulus for a region already facing multiple mine closures.
The extension plans are presently before the planning commission that began public hearings on Thursday.
A similar case involving the Mount Pleasant mine successfully argued the commission must consider the emissions from burning coal overseas on the local community accommodating the mine, as well as pollution generated locally.
The miner is now appealing that decision in the High Court.
Muswellbrook Shire Council Mayor Jeff Drayton was in favour of the HVO extension to give the Upper Hunter region more time to prepare for the transition away from coal mining.
Together with the potential closure of the Mount Pleasant mine, the local community would face "economic and social shock without precedent", he told the hearings.
"Muswellbrook Shire Council is not asking this commission to keep this region dependent on coal forever.
"We are asking for the runway to reposition it."
Governments and industry were working on transition plans but he conceded they were still in the early stages.
Parents for Climate chief executive Nic Seton said approving the mine would not protect Hunter Valley families when global demand for coal was in decline.
"Every year that deepens this valley's dependence on coal is not a year building what comes next," he told the hearing.
"The new industries, the skills, the investment that would let Hunter kids build right here, well beyond this mine - that's the security these families deserve."
The hearings also follow the NSW Net Zero Commission warning that continued coal expansions were not compatible with the state's legislated climate targets.
HVO general manager Dave Foster said the project had undergone six years of scrutiny and was compliant with Commonwealth and state government policy.
"We've made numerous changes to minimise the impacts and meet rapidly changing policy on greenhouse gas emissions," he said.
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