Changes have been approved to a water conservation order for Lake Ellesmere, south of Christchurch.
Environment Minister Nick Smithsays this will allow the Kaitorete Spit between the lake and the sea to be breached more regularly so lake water can be flushed out.
It also puts restrictions on the amount of run-off allowed into the lake from farms.
The minister says the next step is for Canterbury Regional Council to consult the public and it will be the middle of next year before any changes are enforced.
Dr Smith says the change to the conservation order is only one of the measures needed to clean up Lake Ellesmere, which is described as the second most polluted lake in the country.
Lake Ellesmere is 20,000 hectares in size, the fifth largest in New Zealand, according to the Department of Conservation.
The Wises Guide says its depth averages only three metres. It is notable for many waterfowl, duck, swan and pukeko.
Wises says the lake is named after Lord Ellesmore. The Maori name is Waihora (Extensive water)