With specimens dating back to Captain Cook's maiden voyage, the Western Australian Herbarium is something of a botanical time machine.

Key points: The herbarium houses some of Australia's oldest recorded plant specimens

The herbarium houses some of Australia's oldest recorded plant specimens The most valuable samples are kept in a sophisticated fire-proof vault

The most valuable samples are kept in a sophisticated fire-proof vault Its 800,000th specimen was collected by retired couple Geoff and Ruth Byrne

The centre recently catalogued its 800,000th find and, to mark the occasion, invited the ABC inside for a rare look within its walls.

It is an intriguing building, housing row upon row of small red boxes.

Each box holds meticulously preserved plant specimens, hand-sewn onto paper and overseen by curator John Huisman.

John Huisman is responsible for hundreds of thousands of specimens at the WA Herbarium. ( ABC News: Hugh Sando )

"You can look at materials that were collected 200 years ago," he said.

One such specimen is a eucalypt branch, collected by Captain James Cook's botanist, Joseph Banks, on the Endeavour River in Far North Queensland in 1770.

"These are the oldest ones that we have in the herbarium," Mr Huisman said.

The eucalyptus specimen was collected by botanist Joseph Banks in Far North Queensland. ( ABC News: Hugh Sando )

'Vault within a vault' protects most valuable specimens

The herbarium's most important specimens are held in a fire-proof vault to keep the priceless samples safe.

In the event of fire, oxygen would be forced from the room to extinguish flames and protect the collection.

"That's sort of the vault inside the vault," Dr Huisman said.

The herbarium's newest specimen — its 800,000th to be precise — was collected by volunteer research assistants Geoff and Ruth Byrne.

Stenanthera pungens recently became the 800,000th specimen entered into the herbarium. ( ABC News: Hugh Sando )

"It's called Stenanthera pungens. It's found only in one place, which is in the Chinocup Reserve, and that's about 180 kilometres north-west of Albany," Mr Byrne said.

In his previous life, Mr Byrne was a paediatrician and head of the diabetes unit at the recently decommissioned Princess Margaret Hospital for Children.

He also trained as a botanist and zoologist, while his wife Ruth worked as a secretary.

Geoff and Ruth Byrne volunteered at the herbarium to avoid becoming bored in retirement. ( ABC News: Hugh Sando )

The formidable retirees cataloguing WA's flora

In retirement, the couple form a formidable team, photographing and cataloguing WA's flora.

"Without this, it [retirement] would have been absolutely boring. We were determined that that wasn't going to happen to us," he said.

Mr Byrne and his wife have collected plant specimens for the herbarium since 2003. ( Supplied: Geoff and Ruth Byrne )

"We'd seen too many of our colleagues retire and be bored.

"If you look at all of the data on early retirement, people that don't plan, then the death rate in that group is pretty high, and we decided that wasn't for us.

"We needed to be occupied and we needed to do things we're interested in."

These days Ms Byrne is a willing accomplice in her husband's botanical endeavours, but she conceded she was not always as enthusiastic.

"I've always been interested in wildflowers and birds, but I wouldn't have said it was my passion when we were married," she chuckled.

The herbarium is not usually open to the public, but if you are as interested as the Byrnes, there is one way inside — you can always apply to volunteer.