Professor Spoonley, quoting figures from another professor, said that the equivalent excrement from 43 million humans was being pooped on to New Zealand farms every day without a sewage system.
''The way we use farming is unsustainable.''
The global head of agribusiness for KPMG Ian Proudfoot said things had been pushed too far. New Zealand had to farm to the true capacity of its land, and to levels where the community felt comfortable.
He said milk production had peaked in New Zealand and production was on a downward curve because fewer cows were better for the environment.
''I see that farmers are looking at their farms and recognising that they have been trying to extract too much from the land.''
He predicted New Zealand's annual milk production would drop from 22 to 23 billion litres a year to 15 to 17 billion litres.
Hurunui District Council deputy mayor Marie Black highlighted the ''Proud to be Farmer'' campaign, saying most farmers were choosing to leave their land in a better place than when they found it.
Spoonley said there was a growing urban/rural divide in New Zealand, with quite a degree of hostility building up between the two communities.
He noted this particularly during a bicycle tour when he quickly decided not to tell people that he was from Auckland.
Proudfoot highlighted how each community relied on each other, saying Auckland was the biggest food processing centre in New Zealand.
Conference MC and broadcaster Wallace Chapman challenged the panel by saying that New Zealand was destroying its fertile land by bad planning.
Proudfoot said vegetable growing land like at Pukekohe and Pokeno were coming under the most pressure.
Planning Institute chairman Bryce Julyan said in the lead-up to the conference that planning for the long-term sustainable future of New Zealand's cities and resources required immediate attention.
Rapid urbanisation, water quality, and the impact of natural hazards and climate change were among other issues tackled at the conference.
