In an unassuming suburban Brisbane warehouse lies one of Queensland's safest vaults — it contains people's last wishes.

If a fire alarm is triggered, anyone inside the vault has just 90 seconds to get out before the door seals and a special gas replaces oxygen and smothers flames.

"We believe that this is the most secure bunker that you can have," Queensland Public Trustee Peter Carne said.

The stronghold keeps the last will and testaments of 1.2 million Queenslanders in the care of the office of the state's Public Trustee.

Queensland Attorney-General Yvette D'Ath said "that's people's lives".

"That's history and it's also one of the most precious documents to make sure that your wishes are fulfilled," she said.

Will number three, written just weeks after the establishment of Office of the Public Trustee of Queensland. ( ABC News: Liz Pickering )

The free service was set up at the height of the Great War to write wills for those being shipped to the front line.

Every will made out with the organisation is kept.

"You pull out a will and you read it and you can feel the emotion," Mr Carne said as he opened a box and slides out three age-worn wills, numbered 3, 4 and 7.

Will number 3 is a young man from Gin Gin.

"He has here his occupation as farmer, now a member of His Majesty's Forces and poignantly, he leaves everything to his mother," Mr Carne said, reading the century-old handwriting.

Wills 3, 4 and 7 were written at the RNA Showgrounds where soldiers were posted before being shipped to the frontline. ( ABC News: Liz Pickering )

"We were at the height of the first world war and the T. J. Ryan government saw there was a need for those young men and women who were about to leave these shores to fight in the Great War had the opportunity of having a will done," Mr Carne said.

Until six years ago, this and other wills handled by the Public Trustee were stored in 16 offices around the state.

But in 2011, devastating floods were followed by one of the most intense cyclones on record — Yasi threatened Cairns and the 120,000 wills kept there.

The new bunker was given priority.

Shelves holding legal documents held in relation to executing the wills of Queenslanders. ( ABC News: Liz Pickering )

"It's fully engineered to ensure we can protect against cyclone against flood and against fire," Mr Carne said.

The bunker has space for just over 2 million wills. ( ABC News: Liz Pickering )

The atmosphere is strictly controlled and the archive boxes are designed to last more than 500 years.

Long before then, there will be digital wills and a virtual bunker, but the importance of planning ahead will not change.

Ms D'Ath said it was critical to ensure people make their intentions clear for those left behind - and that includes a will, a health directive and an enduring power of attorney.