"Construction is very hard and demanding work, the longer you work the more likely you are to wear your body out.
"The length of the working day is important to people's health, and we need a law back."
Clarke said an eight hour working day was a hard won right that shouldn't be taken for granted.
"If you think about it, a lot of people were killed in America in the late 19th century, trying to get an eight hour day.
"And Parnell created it here in 1840.
"It was lost in the Great Depression, people had to go on strike to fight for it again, and it was lost in 1990, when the law requiring an eight hour day was repealed.
"Lots of people now don't work it, and have to work long hours for their pay."
The Wellington Trades Hall Incorporated was granted $15,000 in 2015 and $50,000 in 2017 for earthquake strengthening, through the city council's Built Heritage Incentive Fund.
Wellington Mayor Justin Lester said it was good to have the Samuel Parnell plaque back in its rightful spot.
"Samuel Parnell was integral in the move to an 8-hour work day around 1840 in New Zealand, so it's significant and fitting that his plaque is going back to be on display at Trades Hall as it opens its doors again.
"We can largely attribute the current 40-hour week practices and Labour Day to his efforts."
The Trades Hall was built on its Vivian St site in 1927, as a base for private sector unions.