These are no ordinary frogs - and that's not just because of their size. These are Romer's tree frogs, a species found only on three Hongkong islands, Lamma, Lantau and Chek Lap Kok.

When the airport bulldozers rolled across the tiny island that is their main habitat, their future was endangered. But now, thanks to a joint project between Hongkong zoologist Michael Lau and the Melbourne Zoo, the frogs have been rescued from the brinkof extinction.

When the World Wide Fund for Nature (WWF) in Hongkong realised the new airport threatened the future of the tiny frogs, it sent out an SOS to the body which oversees the conservation of rare or threatened species, the International Union For Conservation(IUC).

In August 1991, the IUC passed on that call to Chris Banks, an IUC member and curator of reptiles and amphibians at the Melbourne Zoo. By last April he was camped out on Chek Lap Kok catching frogs.

''Michael Lau of the WWF was concerned about keeping all the frogs in Hongkong and was aware there was not a lot of interest or expertise there,'' Mr Banks said.

''He wanted help to take a colony of frogs from the island and establish another colony elsewhere - to breed them and then send the frogs back once he had found new suitable sites for them.'' Mr Banks won permission from Qantas and a special dispensation from the Civil Aviation Authority to carry his container of fragile and tiny frogs in the passenger cabin.

Armed with the necessary permits, he and Mr Lau, who says it's dangerous to keep all the frogs in one place in case of an accident or outbreak of disease, spent a night on Chek Lap Kok catching frogs for Melbourne.

''The male frogs start calling after dark, that was about 7pm, so we wore hats with headlamps like miners, zeroed in on a calling frog and caught it. We hoped that others we saw and caught that weren't calling were female,'' said Mr Banks.

The result, supplemented by 10 frogs from Mr Lau's supplies, was 11 females and 19 males, all of which survived their journey Down Under.

So while Mr Banks set about raising their offspring, Mr Lau, based at Hongkong University as a PhD student and with a HK$250,000 Royal Hongkong Jockey Club grant, has been studying the frogs breeding and genetics and seeking new sites for them to live.

The display tank, which contains seven frogs, is supplemented by another 19 in the reptile house breeding area. Each contains 10 to 15 frogs and is covered with gauze beneath its green lid to keep in the tiny fruit flies the curators breed as frog food. Other water-filled tanks contain several hundred tadpoles.

''Some of the female frogs were full of eggs and started to spawn within a month of arrival. Probably 400 or 500 metamorphosed to froglets but only about 60 survived,'' said Mr Banks.

''They are so small they are susceptible to environmental changes - rises in humidity, toxins in the soil - in a matter of hours.

''Late November we started getting calling again from the frogs, including some we have raised, which was really exciting stuff. Now we have spawn from both groups and we are keeping it separate to look for any difference.'' Mr Banks said two possible new sites for the frogs have now been found, one on another island, the other in the Hongkong Botanic Gardens. If trials are successful, Mr Lau plans to visit Melbourne later this year and take back some of the fast breeding Aussie colony.

None will be destroyed and most will eventually go back to Hongkong. ''We cannot maintain hundreds of one species of frog ad infinitum, especially as there are 200 to 300 eggs each time one female lays,'' said Mr Banks.

The zoo may keep a small colony here if asked to by the Hongkong end of the project. Even if it doesn't, the staff have learned valuable lessons that will be put to good use when the zoo's own specialist frog centre is built next year.

''It's not enough just to have these creatures on show,'' said Mr Banks. ''It's vital that we work with Asian neighbours such as Hongkong to save those that are threatened.''