Sewage-contaminated water will continue to be discharged into unlined ponds in Bottle Lake Forest for years, Christchurch City Council says.

Residents living near the Burwood Resource Recovery Park report seeing up to a dozen trucks a day disgorging tens of thousands of litres of liquid waste into the earthquake dump site.

People on the beach about 100 metres away have also contacted The Press with concerns about the smell from the ponds and the risk of contaminants leaking through the sand dunes and on to the foreshore.

However, the council says the small settling ponds, which are about a metre deep, are environmentally safe and adequately monitored.

Forest Park resident Leigh Harris said a pond was a "hole which is nothing more than a giant sandpit just 100 metres from the beach".

The liquid "obviously included raw sewage because when the wind blew in the right direction the area smelt like a pig farm".

"You don't need to be a rocket scientist to work out all that liquid will seep through the sand and out into the bay," she said.

"Last summer I mentioned to a couple of people swimming that there was a virtual lake of sewage just behind the dunes from where they were and they were pretty horrified."

Council water and waste manager Mark Christison said yearly environmental monitoring was carried out on the pools, which were built after the February 22 quake.

The latest testing had been undertaken in "about March-April" this year.

The ponds had been dug in a location where "they can't affect groundwater", Christison said.

There were no signs of any contamination, he said.

Trucks were discharging wastewater containing sewage and stormwater with sand and silt in it.

"A lot of the stuff is coming from badly damaged eastern parts of the city," he said.

"We can't take it out to the [Bromley] treatment plant because there's too much sand in it."

A small pumping station had been built at the site, Christison said.

"That takes the liquid off the top of the material and pumps it back to the wastewater plant at Bromley for treatment. That leaves the silt and sand, which gets excavated out and put into the landfill and covered."

Fewer trucks were dumping water now than immediately after the February 2011 earthquake, he said.

"Until we get sewer catchments repaired, we will still keep trucking," he said. "It'll be a couple of years yet."

Harris said the smell was not as bad as it used to be. She had not seen any solid waste being excavated from the main pond.

"I don't have a problem with the council having to do things a little differently post-quake, but as their handling of the asbestos issue showed, it's simply a matter of communication. Please just tell us if it's safe or not."

- © Fairfax NZ News

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