Napier's industrial area of Awatoto will be cordoned off for at least a week because of contamination.
Napier City Council says testing has found high levels of contamination in silt and water left by last month's flooding. It includes traces of acids, pesticides and caustic agents.
"We've had an initial report from the scientific and technical advisory committee identifying that there is the potential for some significant contaminants in the area," Napier Mayor Kirsten Wise told Morning Report on Wednesday.
"Our next step is doing further testing to actually ascertain exactly what they are."
The council said security guards would enforce a hard cordon, which will be in place until at least Thursday next week.
There are 20 businesses in the area, and specialists are considering how to safely remove the contaminated silt and water.
"It's primary primarily, you know, industrial-type businesses," Wise said. "Ravensdown fertilisers, a tannery, Higgins has a site there, BioRich, Hawke's Bay Protein - you know, quite a complex mixture of different types of businesses."
The council said public health risks for a nearby residential area are low.
"Public and community safety is absolutely the first priority and we do have a hard cordon in place," said Wise.
"We're also really conscious that the business owners need to be able to continue with their clean-up. They are an important part of our local economy, so we are working with them to ensure that before they gain access to their sites, they've got the appropriate safety mechanisms in place, including PPE, safety plans, etc.
"So in terms of the general public, however, there's no access to the site whatsoever, and we're also working with the nearby residential areas to ensure that they are being kept safe."
The council is "seeking answers" on who will foot the bill for the clean-up, Wise said. Only some of it is council land, parts of it privately owned.
"Currently, it's individual business owners who are doing the clean-up. Obviously, if we are getting some assessments back that identify there is significant risk, then we would need to liaise with the relevant government agencies in terms of seeking advice from them around the next steps."
In the meantime, labs will be testing samples from the site to find out exactly what is in the leftover silt and water.
One employee described maggots amongst piles of silt and mud, with damaged goods being piled on the roadside.
One business, Ravensdown Napier, confirmed there was no sulphuric acid leak as their acid and fertiliser manufacturing lines remained above water at the site.
Their admin and lab buildings were not so lucky - they were significantly damaged, but works manager Andrew Torrens said overall it was not as extensive as nearby businesses.
"We are working closely with Napier City Council and are encouraged by their approach to working collaboratively with all businesses in the Awatoto Industrial Area," Torrens said.
"Before the cordon was put in place we had made considerable progress in terms of assessment and recovery.
"While a return to normalcy is some way off, we have managed to implement a number of workarounds to help meet customer expectations."
Ravensdown was waiting on sample results after initial testing showed traces of fertiliser nutrients, he said.
There were currently no issues with the supply of fertiliser, Torrens said.
The other route into the industrial zone via Awatoto Road is closed, and the corner with Eriksen Road has become a dump for homes impacted by floodwaters and the muck left behind.
Nearby residents and businesses told RNZ they felt ill-informed about the contamination risk, and they should have been told more.
Wise said she understood their concerns and the council has been sharing information as it came out.
"We have had welfare officers in the area knocking on doors to talk to them. So we are managing that risk and we're working through the clean up as promptly and safely as possible.
"We have got that standing offer there for all the residents in that area to provide alternative accommodation for them if they require it."
Samples are being tested in Awatoto to find out what exactly is in the leftover sludge.
A community meeting is being planned for residents on Thursday.
Hawke's Bay Regional Council has contracted silt samples to be tested from the Esk Valley south to Twyford and the surrounding area.
The aim is to find out what is in the silt including possible contamination from chemicals, animal waste and localised oil or diesel from vehicles caught in the flooding
The initial feedback - which did not include Awatoto - suggested there was nothing significantly contaminated, but people should still wear suitable PPE as a precaution.
The regional council said they would use the results to support the silt to be safely removed and stored, and would coordinate this for farms and orchards only.
Roads reopen
Parts of two major roads in northern Hawke's Bay have reopened as efforts are made to reconnect the region.
Waka Kotahi said two sections of routes into Wairoa have reopened after being assessed by geotechs, with State Highway 2 from Raupunga and State Highway 38 between Wairoa and Frasertown opening earlier this week.
Regional maintenance and operations manager Jaclyn Hankin said motorists should not expect highways to look as they did before the cyclone.
A locked gate will also be installed at the top of the Mohaka Hill on SH2 to prevent people accessing Waikare Gorge.