Cows are not blocked from wading into Lake Ellesmere, despite rules forbidding stock from accessing natural waterways.

A decades-long effort to bring the country's most polluted lake back to life is in its early stages, but not all are committed to the ambitious project.

Lake Ellesmere is being restored after the troubled lake was declared "biologically dead" in 2005. Current data shows the lake's water quality is continuing to decline.

NIWA A satellite image of Canterbury from 2012 shows the extent of pollution at Lake Ellesmere.

The restoration – backed by groups including Environment Canterbury (ECan), the Department of Conservation (Doc) and Ngai Tahu – is expected to take several decades and could cost up to $200 million, outgoing ECan commissioner Donald Couch said in May.

About $10 million has already been spent on regulatory changes and replanting lost weed beds with hundreds of thousand of plants.

Images taken on September 12 show stock grazing on the shore of the lake with no barrier stopping them from wading into the water.

SISSI STEIN-ABEL A grey, sludgy foam washing up on the edges of the lake.

Farmers are required to prevent their stock from accessing natural waterways.

The pictures, taken by freelance journalist Sissi Stein-Abel, showed the lake looking "horrible as never before" with flies swarming the shore and a greyish sludge forming on the edges, she said.

She sent the photos to Environment Canterbury, which is investigating.

SISSI STEIN-ABEL This photo, taken in the same area in 2012, prompted ECan to investigate.

ECan monitoring and compliance manager Marty Mortiaux said the authority would speak to farmers nearby to remind them of their obligations.

"We did a lot of work with farmers in the catchment about a year ago to prevent stock grazing around the lake edge and thought we'd got the message out, but obviously now we need to follow it up."

ECan investigated images showing cattle wading in the lake in 2012. It took no further action as the farm had been de-stocked but reminded landowners of their obligations.

The landowner has not been identified. There are more than 800 rural property parcels along Christchurch Akaroa Rd.

The images were an example of a broader problem facing the restoration effort, said Ken Hughey, professor of environmental management at Lincoln University.

Hughey – a co-founder of the Waihora Ellesmere Trust, a community group coordinating efforts to save the lake – said the project had struggled to get full commitment from some groups.

"A lot of people see a greenish, brownish-tinged lake at the foot of the plains and don't see the really diverse values the lake holds," he said.

"There's a real challenge getting that buy-in."

Although attitudes were slowly changing, some people, as shown in the photos, were not fully committed to the effort.

Whether water quality would improve, even 20 years from now, remained to be seen.

"I think we have the bulk of people pretty much on-side now. There will always be one or two late followers who are easily picked up when we see those images.

"We do need to engage with them. But in my view, attitudes are changing hugely."

Surface water scientist Tim Davie said the clean-up project was in its early stages and it would be some time until improvements were seen.

"All of the partners involved... acknowledge that this is a long-term project that will take decades rather than years to achieve," he said.

"At this stage, we're not seeing huge improvements in water quality from the nutrient rules but we are in terms of habitat restoration and we continue to support these initiatives."

Federated Farmers mid-Canterbury provincial president Chris Allen said landowners should check their obligations with ECan to see what rules applied to their stock.

Timeline:

2005: Lake Ellesmere declared "biologically dead" by Environment Court judge Jeff Smith.

2007: Studies show the lake is not dead. Rather, parts of it are "terminally ill" while others are salvageable.

2011: Government announces $11.6 million clean-up plan, which involves replanting weed beds and changes which give stronger protections to the lake.

2015: ECan introduces environmental restrictions on nearby farmers, including reduced takes for irrigation, reduced nitrogen discharges, and a requirement to consult Ngai Tahu before getting resource consent as part of the clean-up effort.