One of the organisers of a seabed mining protest at Castlecliff beach on Tuesday afternoon is promising further action.

About 400 people turned up to the protest, which included a blessing, speakers, a human chain along the beach, and plenty of colourful banners.

Te Huatahi Hawira, of Ngā Rauru, organised the protest with the help of a friend.

"It went really well. I'm pleased so many people turned up. Hopefully this will lead to other things - this is only the beginning," Ms Hawira said.

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"I want to thank everyone for caring."

Ms Hawira organised the protest to give people in Whanganui the opportunity to let their opinion on seabed mining be known.

The Castlecliff protest follows several others in Patea over the Environmental Protection Agency's decision last month to allow Trans Tasman Resources to mine ironsands off the south Taranaki coast.

Whanganui District councillor Jenny Duncan went to the protest because she was "horrified" by the EPA's decision.

"I made a submission against the mining, and I am fully in support of this protest. We have to send a strong message that this is not okay."

Castlecliff resident Graham Pearson also made a submission against the mining.

"They didn't listen to me, so I decided to come to the protest. Any seabed mining won't just affect south Taranaki - it will affect Castlecliff too."

Teacher Teina Aue accompanied the 100-plus group of children from nearby Te Kura Kaupapa Māori o Tupoho at the protest.

"The moana (sea) is right on our doorstep, and if they mine at Patea that will be our food basket gone."

Ms Aue said the students at the kura had been recently learning about issues around seabed mining.