Botanists would prefer to preserve plants in their own countries, but until that's possible she hopes the PlantBank could also provide a way of preserving a large amount of rainforest floar under threat in countries like Papua New Guinea and the Solomon Islands. The paper estimated that more than 80 per cent of threatened rainforest plants did not exist in more than one country.

Speaking at the botanical gardens to mark Science Week, NSW's environment minister Gabrielle Upton said the research was the first comprehensive review of rainforest seed conservation in the South Pacific.

The gardens' plant scientists were "delivering real solutions to secure the future of rainforests in the South Pacific while contributing to international conservation goals", she said.

With rainforests rapidly disappearing because of logging, clearing and pests, the research estimated that 95 per cent of 386 genera (groups of related species) may be suitable for seed banking.

But there were more than 790 genera that scientists knew nothing about, said Dr Sommerville in the paper on Saving Rainforests published in the Journal of Botany.

"There is quite a bit of work that needs to be done to figure out if we can put those seeds in the seed bank," she said.

The gardens' National Herbarium in Sydney contains a historical collection of 1.4 million plant specimens with the earliest dating back to Joseph Banks and Daniel Solander on the Endeavour. The PlantBank was established in 2013 to provide an insurance policy for plants at risk of extinction. It now contains more than 5130 species and 60 per cent of NSW's threatened species.

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Since then,  it has discovered that seeds weighing less than 20 milligrams can be frozen and preserved.  "That helps us to narrow down the species that we need to focus on," said Dr Sommerville.

Before seeds are frozen, they need to be dried. "Some will take drying but not freezing. Some are fine with drying and freezing, but some won't take either," she said.

Even within groups of similar plants there were differences that made seedbanking difficult. Some families of plants like the laurels won't tolerate drying, they've found.

Rainforest plants with large seeds very often won't survive drying and freezing, said Dr Sommerville.

For example, the hairy walnut and the black bean, which both have large seeds that fit in the palm of a hand, don't like to be dried.

Some oily seeds are hard to preserve, and wouldn't tolerate freezing, but the macadamia nut has proved the exception.

Only last month a researcher found the macadamias were germinating after being stored in minus 192 degrees celsius in liquid nitrogen vapour for a month.

Rainforests contribute to a substantial amount of biodiversity, accounting for less than 0.5 per cent of Australia yet are home to 5500 plant species.

Science Week runs until Sunday 19 August with hundreds of free events at universities, libraries, the Australian Museum and the Gardens.