"When they got a local electrician in to check the house out, he put his ladder up on the roof and got about three steps up before he was thrown flat on his back."
Mr Seamark noticed "alarm bells going off" as he listened to a recent speech by Go Underground Waikanae chairman Don Little, about the effects of electromagnetic fields on people's health.
He wondered if other Waikanae families had similar stories.
With 1959 bringing the Thomas family's first cancer battle, their most recent incidents happened last year, seeing Mr Seamark's mother-in-law die from multiple myeloma in July, followed by her son's death from cancer of the liver, bowl and bladder five months later.
Among the family, there has so far been three cases of multiple myeloma, two cases of breast cancer and one leukemia death.
"They'd all lived in that house under the pylons and some were born there.
"I think there's a definite link because there was no history of cancer in the family prior to this.
"They were long livers."
Go Underground Waikanae member Tony Lester said there was growing evidence in world health studies pointing to the detrimental effects of electromagnetic fields.
"Transpower states that these things are within the national guidelines, and they may well be.
"We're not scientists who are going to battle them over this.
"All we're saying is this is a family that lives in close proximity and has had instances of cancers they'd never had before living below the pylons."
¦Go Underground Kapiti asks if anyone has a story, to contact them via their website or email liddle.don@gmail.com