Times are changing, Dr Tanner said.
Some people have always found wetlands attractive - as hotspots for biodiversity and sources of food - eels and ducks - and fibre - especially harakeke (flax).
They're inspiring a new wave of interest now because of the "ecosystem services" they provide. Dr Tanner listed them.
Swamps and ponds can slow the rush of water entering rivers in times of heavy rain, reducing the flood peak and spreading it over a longer time.
They can make waterways cleaner and clearer, by trapping silt before it gets to them.
Wetland plants drop into the water when they die, and decay slowly. This creates methane, a greenhouse gas, but the overall effect is to limit climate change by storing carbon over a long period.
Most importantly, at a time when too much nitrogen and phosphorous is leaving farms and affecting water quality, wetlands can remove some of these nutrients.
Nitrogen leached from farmland is usually in the form of nitrate, NO3. In wetlands the bacteria breaking down plant material are often short of the oxygen they need. They grab the oxygen content from nitrates, and release the nitrogen back into the air as a gas.
Phosphorous particles and sediment get physically trapped by plants and biofilms of microorganisms, in a process that's quicker than the slow breakdown of nitrate.
Wetland reeds and rushes also absorb nutrients and, because they are fibrous, they release them only slowly when they die.
Nutrients and silt enter waterways faster, and in larger quantities, when land is drained. A catchment that is 1-5 per cent wetland can capture 30 to 50 per cent of leached nitrogen, and 50 to 75 per cent of sediment and phosphorous, Dr Tanner has found.
The quantity captured depends on things like temperature - it's slower in winter - and how long the water stays in the wetland.
The ability of wetlands to remove "contaminants" is starting to be recognised by regional councils. Dr Tanner said Horizons Regional Council was ahead of others on this.
People wanting wetlands on their farm were advised to choose places where water already sits. They could be former swamps, and still have reeds and rushes. They could be the last wet areas on the farm, and would otherwise be expensive to drain.
They can simply have drains filled, be fenced from stock, and be planted with suitable plants. Constructing a wetland where there was none before is much more expensive. It costs about $100,000 per hectare, Dr Tanner said.
Some of the landowners present were already enthusiastic about wetlands. One man had a very small farm with nine of them. He loved the wildlife they generated.
"They're a fun thing to have on your property. Anyone who has one will tell you," he said.