Eleven speakers have been lined up, including Ngati Kahungunu Iwi chairman Ngahiwi Tomoana, Bruce McKay of Heinz, Federated Farmers president Katie Milne, local government figures, and political candidates.
Rain fell on about 20 people taking part in the walking rally from Waitangi Estuary.
Among them were Havelock North's Peter and Margaret Watt, who were driven to take part out of concern for how the WCO could impact the region's primary sector.
"I'm really ticked off that someone from out of the region thinks they can tell us what to do with our water," Mr Watt said.
Mrs Watt said she thought the TANK process was a much better way to protect the waterways.
Eastern District Police advised motorists to expect delays if they were travelling on State Highway 2 between Hastings and Napier this afternoon.
"We ask that motorists try to avoid travelling in the area during this time, however if it is necessary then please allow extra time for your journey."
The same has been echoed by the New Zealand Transport Agency, which has advised motorists to take alternative routes from midday to mid-afternoon.
People who set out to travel along that stretch at that time can expect to add about 10 minutes to their journey.