The largest seed bank in the southern hemisphere is being constructed in Sydney's south-west to help prevent a loss to the planet's diversity of flora.

Scientists are collecting and housing a range of plants and seeds to fill Mt Annan's PlantBank, which is being built under the management of the Australian Botanic Gardens.

It will eventually house up to 25,000 plant species not only from Australia, but from across the Asia-Pacific.

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Project manager Brett Summerell says the PlantBank will be a form of "ultimate insurance".

"If something calamitous happens then we're able to extract the seed from the seed bank, re-grow them, and in some cases re-vegetate areas so that the plants are still existing," he said.

The bank will include a super-sized refrigerated vault to house trays and envelopes full of seeds.

Mr Summerell says the seeds are sealed under a vacuum so that they will last a long time.

"We've just been discussing how long these seeds are among the research group here, and the tests that we've done so far indicate some of these seeds could survive for up to 6,000 years under these sorts of conditions," he said.

Global initiative

The United Nations Environment Program identifies Australia as one of 12 "mega diverse" countries and fifth in the world for its mega diversity of flora.

Mt Annan's PlantBank follows similar initiatives around the world, including the Svalbard seed vault in Norway.

Mr Summerell says the Svalbard's seed bank is built right into the permafrost.

"It's not conditions that we can actually mimic too well here, but certainly the whole process of seed preservation and collecting - we share information with organisations like that and also with the Royal Botanic Gardens in Kew in the United Kingdom," he said.

The Mt Annan bank will be used to research the best conditions to keep different types of seeds.

"For example, rainforest seeds in northern New South Wales and Queensland are very, very difficult to preserve because they're fleshy fruited, usually very short-lived, so we want to develop techniques and conditions, maybe using liquid nitrogen - cryopreservation - so that we're able to store species like that as well," Mr Summerell said.

"Also, we want to start to work more in the other end of the seed banking program which is not just keeping the seed - having a collection - but also getting the seed out of the collection so that you can bring restoration programs and re-vegetation programs for threatened communities both locally and across the country."

Threats to diversity

Mr Summerell says land clearing, temperature changes and plant diseases are the biggest threats to plant diversity in this region.

"There's a new disease - myrtle rust - which is likely to have a very, very big impact on those species. They seem to be very susceptible and likely to be very adversely affected," he said.

"We're really wanting to focus on those species and make sure we have them in the seed bank in case something particularly unpleasant happens with this disease.

"And these plant communities are also under threat a lot from the impact of land clearing and urbanisation in the northern New South Wales coastal regions, so they are important plant groups that we want to focus on."

Mt Annan's PlantBank will open at the end of this year.