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Engineers from Opus International and representatives from the New Zealand Historic Places Trust (HPT) arrived in Oamaru this week to assess options for the courthouse, which was deemed an earthquake risk last November.
However, local property investors have now also taken issue with the classification of Oamaru stone as unreinforced masonry, which could increase earthquake-strengthening costs for many of the town's heritage buildings.
New Zealand Property Investors Association North Otago branch president Helen Stead said building owners were faced with uncertainty over earthquake-strengthening levels, increased costs for insurance, low valuations and "forced closure of certain public buildings".
"Of particular concern is nervousness about risk, with legal ramifications the driving force, both in received costs for upgrading as well as alarming increases for insurance cover on Oamaru stone buildings and structures.
"In almost all cases, costs for strengthening local heritage buildings have been considerably lower than originally stated by national bodies. Similarly, the sharing of information about local conservation methods needs to be more widely known and appreciated."
Mrs Stead said a seismic and structural engineer with international experience rebuilding earthquake-damaged buildings would visit Oamaru and give a presentation on his thoughts on the use of Oamaru stone next Tuesday .
The Ministry of Justice commissioned Opus International to carry out further investigative work earlier this month, after the ministry's original estimate of how much it would cost to strengthen the historic courthouse was questioned by Waitaki Mayor Alex Familton.
The ministry had claimed strengthening would cost $5 million, but Mr Familton said it could be done for a tenth of that.
A Ministry of Justice spokesman said the latest work, which would include taking geotechnical and core samples, would "take some time" to complete.