Cyanide and heavy metals unearthed during Invercargill's Esk Street redevelopment threatens significant disruption and further delays to the inner city upgrade.

Work was halted last week after the discovery of an unknown substance amongst archaeological material and excavated soil.

Council Group Manager Infrastructure Erin Moogan said the items were tested and early results had identified low levels of cyanide and a mix of heavy metals in the materials.

“Work has now been stalled as we undertake further investigations and testing but the community can feel confident that the identified contaminants in Esk St do not pose any risk to public health,” she said.

“The level of contaminants is low and we have strict health and safety measures in place. We’re really proud of how quickly and cautiously our teams have acted.

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“Environmental consultants are on site to undertake further testing and examination and we are working through the processes and environmental consent requirements to ensure any contaminated materials are handled in the appropriate manner.”

It is likely that this process and subsequent procedural requirements will mean works will be significantly disrupted until some time in January, and that the project and milestone dates may also be extended due to this delay, Ms Moogan said.

“In a site such as Esk St, which is a long-established urban roadway, contaminants can sometimes be found in materials previously used to construct roading surfaces, such as coal tar, which was used to provide a hardwearing roading surface before the advent of asphalt.”

It is hoped the work will be able to commence as quickly as possible, she said.

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Shoe Clinic owner and chairman of the Neighbourhood Retail Group, Ben Fokkens said it was obviously unfortunate this had popped up but it was something that can't really be planned for.

He was hopeful that [construction] could get back on track, maybe with extended hours and weekend work.

Getting the road into a usable condition was a priority but he was grateful that H&J Smith had opened their carpark to Esk Street customers.

In the meantime he has started offering free home visits and drop offs.

McKnight and Brown's Bernie Brown also hoped developers could make up for lost time, saying it would be a huge boost to retailers to have vehicle access back for Christmas 2022.

Southland Business Chamber CEO Sheree Carey said development was already on a tight schedule to finish before Christmas next year. She said [the delay] was a real shame but no ones fault.

Carey said the chamber was there to support both retailers and the CBD at this time.