Almost 40 years after politicians abandoned it for good, the musty corridors of Canberra's Old Parliament House will live again. 

In April 2027, 226 MPs and senators, along with their staff and a gaggle of journalists, will trudge down Capital Hill for a one-day sitting to celebrate the building's 100th birthday. 

A century earlier, King Charles III's grandfather - then the Duke of York and later King George VI - opened "Provisional Parliament House" with a big golden key.

Back then, the building stood almost alone in a barely developed patch of countryside.

Flocks of sheep still roamed the paddocks where Lake Burley Griffin now lies, though it would not be filled with water for another 36 years. 

Old Parliament House was built to be temporary, but it would be the backdrop to some of Australia's most spellbinding political moments until 1988.

The most famous of these was the 1975 sacking of prime minister Gough Whitlam, whose speech on the building's steps is etched into national folklore.

The Aboriginal Tent Embassy, erected on its front lawns in 1972, is thought to be one of the longest-running protests for Indigenous land rights in the world. It remains there still.  

The former parliament now hosts the Museum of Australian Democracy.

"Learning from our democratic history and institutions is more important than ever," Prime Minister Anthony Albanese said in a statement on Friday.

"Our unbroken democracy goes to the heart of who we are as a nation and why we continue to be a beacon to the rest of the world."

Complaints about the provisional building being almost unworkable and cramped were part of what prompted the move to the current Parliament House in 1988. 

Cramming today's politicians, none of whom worked in the old building, will certainly be a squeeze, but there are only three more lower house members than there were when parliament relocated.

The number of senators also has not changed since 1988. 

Celebrating Old Parliament House's 100th anniversary will cost $3 million in total, according to May's federal budget, with arrangements for the special occasion being made by a special parliamentary committee.