Despite the hefty price tag the Greens believe cleaning up a Canterbury lake dubbed a health hazard will be money well spent.

Lake Forsyth has been declining for decades, but was official declared a hazard in January after a blooms of toxic green algae began accumulating.

Environment Canterbury [ECAN] is now considering removing around two-million litres of the phosphorus rich soil from the bottom of the lake, which could cost between $12 and $30 million.

But Greens water spokesperson Catherine Delahunty said it's worth it.

"We have to look at causes and look at what's going to work long-term, but we can't just throw up our hands and do nothing," she said.

"So I think it's better to spend money now than to leave problems to get worse and then just spend it later.

"This is a bit bigger than just digging up a lake. Rather than just saying ECAN's idea is right or wrong, I think we have to look at what is causing the problem in this lake, and what can be done that will actually address those causes."