Joyce certain he'll get control of water

Barnaby Joyce has played down reports of simmering tensions within the federal coalition over which minister is responsible for water policy.

Prime Minister Malcolm Turnbull said he would shift water from the environment portfolio to Mr Joyce's agriculture ministry.

"I'm absolutely certain that will happen," Mr Joyce told ABC radio on Monday amid reports his assistant minister, Liberal Anne Ruston, would have specific responsibility for water.

Mr Joyce refused to comment on reports Mr Turnbull had yet to reply to his letter requesting Senator Ruston be given responsibility for wine and horticulture.

That's led to some Nationals warning the new prime minister risks undermining the agreement he negotiated with the junior coalition partner a month ago.

But Mr Joyce is not one of them.

"I'm absolutely certain the agreement will be honoured," he said.

Returning water to the agriculture portfolio was a key aspect of the agreement.