The Treasury Department has confirmed they are investigating if its chief used taxpayer money for a personal trip to view the solar eclipse.

A watchdog group believed Treasury Secretary Steve Mnuchin and his wife Louise Linton took a government plane to view the 21 August solar eclipse in Tennessee.

The pair joined Senate Majority Leader Mitch McConnell in his home state to view the event atop Fort Knox, an Army base and home of $200 billion US gold bullion depository.

Mr Mnuchin is the first Treasury Secretary to visit the gold depository in 70 years.

Rich Delmar, counsel to Treasury’s Office of Inspector General, said in a statement that the agency is “reviewing the circumstances of the Secretary’s [21 August] flight to Louisville and Ft. Knox to determine whether all applicable travel, ethics, and appropriation laws and policies were observed.”

The agency’s original response to public inquiries from ethics watchdog Citizens for Responsibility and Ethics in Washington (CREW) and Democratic Senator Ron Wyden was that Mr Mnuchin was in the state to discuss Donald Trump’s tax reform plan.

It also said Mr Mnuchin will reimburse the government for Ms Linton's travel "as is longstanding policy regarding civilians on military aircraft. The trip was originally planned for earlier in August but was postponed to accommodate the Congressional calendar."

Mr Delmar said that when the review is complete, the agency will “advise the appropriate officials, in accordance with the Inspector General Act and established procedures.”

The Treasury Department for financial records showing "authorisation for and the costs" of the trip using a government plane.

In its Freedom of Information Act request, the group wrote: "At a time of expected deep cuts to the federal budget, the taxpayers have a significant interest in learning the extent to which Secretary Mnuchin has used government planes for travel in lieu of commercial planes, and the justification for that use."

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CREW spokesperson Jordan Libowitz told CNBC that the issue would not have caught their attention if not for "the Instagram rant" by Ms Linton.

"That picture was clearly of a government plane, so that raised questions of what the relationship was of her to the trip, and what she was doing there," he said.

Ms Linton posted a picture on Instagram of her and Mr Mnuchin stepping off a government plane the day of the eclipse. In the post the Scottish-born actress described her outfit, naming several high-end fashion designers, calling it a "great #daytrip".

A commenter, Jenni Miller, criticised Ms Linton for bragging about her designer wear while many Americans are suffering financially.

It was Ms Linton's response that also raised suspicions for CREW.

Ms Linton wrote: "Did you think this was a personal trip?! Do you think the US govt paid for our honeymoon or personal travel?! Lololol".

She went on to say in her lengthy response to Ms Miller that the commenter was "adorably out of touch" and said the couple had contributed more in US tax dollars than Ms Miller had.