When the shop was a chemist's the proprietors had covered the mural with a curtain but Mrs Hall said a new tenant wanted to use the wall space, hence the decision to cover the mural with a protective wall.
The mural's future was called into question four years ago when Dunedin artist Bryan James, biographer of Mervyn Taylor, wrote to the Times-Age seeking assurances the building's change of ownership would not mean the mural would be destroyed or neglected.
Mr James said Taylor, who died about 45 years ago, was a major figure in New Zealand art and the country's most important muralist.
The artist's granddaughter, Sarah Taylor, of Wellington, has contacted a Masterton friend, John Allan, expressing her dismay the mural is to be covered over.
Ms Taylor said it was "tragic. Don't they know they are covering up a national treasure, thank goodness it is protected."
The mural is on Masterton District Council's heritage list as a heritage item, but Mr Allan said he had been told that did not prevent it being covered.
A second Taylor mural is in the War Memorial Stadium.