Green Party MPs Catherine Delahunty and Kevin Hague will attend a public meeting in Havelock North to support local concerns about water quality.
The Wednesday night meeting is the second to be held, the first was on Tuesday night when residents called for action over the contamination that has hit more than 5000 people with a gastro bug.
A protest march is to be held on Saturday.
"We're going to show our support for the community of Havelock North and to hear their concerns about the state of the water in their area," the MPs said.
"It's unacceptable that in 'clean, green New Zealand' people should have to endure an outbreak like this."
The Greens have previously called for the Government's inquiry into the contamination to include an investigation of the impact of land use.
Ms Delahunty is the party's water spokeswoman and Mr Hague the health spokesman.