Environmental protesters are trying to shame the prime minister by casting her shadow over one of the country's dirtiest rivers.
The billboard is among three put up by Greenpeace at the Selwyn River in Canterbury.
Greenpeace campaigner Gen Toop says the main billboard features Jacinda Ardern, with her 2017 quote - "we will clean up our rivers."
She said it was time for Ardern to make good on her promises as she finalised the government's freshwater reforms.
Toop said Greenpeace was particularly concerned the government would fail to cut the use of synthetic fertiliser.
Greenpeace said use of the synthetic fertiliser had increased 627 percent since 1990 and in that time the dairy herd has more than doubled.
"Nitrogen pollution from cow urine and synthetic fertiliser run-off threaten the safety of our drinking water and drive algal blooms which kill off life in rivers and lakes" Toop said.
"These billboards are a gentle reminder to the prime minister, that a cap on synthetic fertiliser must be part of the clean water plan. Anything less will render new freshwater reforms meaningless."
She said several studies have shown farmers could increase or retain profitability when reducing synthetic fertiliser and farming fewer cows.
A recent study by Landcare Trust showed farmers could increase profitability by 29 percent through cutting synthetic fertiliser and lowering cow numbers, Toop said.
"A cap on synthetic fertiliser is the first step to phasing out the use of this harmful chemical altogether and putting New Zealand on the path to higher-value and more profitable food production through regenerative farming."