There are concerns building standards at a gas plant construction site in Central Queensland could contaminate drinking water.

Engineering and construction company Bechtel is building three liquified natural gas plants on Curtis Island off the coast of Gladstone.

Gas, water and sewage is piped between Curtis Island and the mainland.

But Brad O'Carroll from the Plumbers Union says the company is hiring pipe fitters to do work that should be done by qualified plumbers.

"It's not only joining the pipes together, it's understanding the whole system - when you have backflow prevention devices, when you need to protect the water with vacuum breakers," he said.

Mr O'Carroll says public health is being put at risk.

"You don't allow water pipes or pipes for the disposal of sewage to be done by unqualified people," he said.

Bechtel says it is in talks with the union and Gladstone Regional Council.

It denies its work is threatening community health.