Conservation groups have joined forces to stop the auction of black rhinoceros horns and elephant tusks in Sydney on Friday, saying the sales will increase demand and consequently poaching, which is decimating the species.

Auction house Lawsons, based in Leichhardt, expects the bidding war for the black rhino horns to hit $70,000; the pair of unmounted African elephant tusks to reach $70,000, and the embellished elephant tusks with a gong  to reach $16,000.

Humane Society International (HSI), with backing from the International Fund for Animal Welfare and Greenpeace, is demanding Lawsons pull the items from auction and change its policies to prevent similar items from surfacing in the future.

"The pressure on the remaining wildlife populations of rhino in Africa, India and [south east] Asia is such that all efforts must be made to stop rhino horn being trafficked," wrote Alexia Wellbelove, senior program manager at HSI, in a letter to Martin Farrah, managing director at Lawsons.

"Even the export of one antique horn from Australia onto south-east Asia markets further promotes and encourages trade, perpetuating this devastating cycle of killing."

Ms Wellbelove said two letters expressing concern were ignored, and in a follow-up phone call last week Mr Farrah told her: "We have nothing else to say."

The world rhino population has dropped from 500,000 at the start of the 20th century to just 29,000 because of poaching, according to the Save the Rhino organisation based in London.

The price of rhino horns has skyrocketed in the past decade because of rising demand from Chinese and Vietnamese people who believe it can cure cancer and be used as an aphrodisiac.

In March, Lawsons sold a pair of rhino horns mounted on a wooden plinth for $92,500 - a figure that shocked antique and auction experts across the country.

Simon Hill, general manager of Lawsons, said the black rhino horns belonged to a Cairns woman who inherited them from her father who migrated from Africa to Australia in 1950.

He said the auction house has contacted the federal environment department to obtain approval for Friday's sale of the 4.6 kilogram rhino horns set and elephant tusks.

Under Australian law, the import and export of rhino horns dated from 1950 is banned and, since July, anyone wishing to export vintage rhino horns must conclusively prove its age through radiocarbon dating.

A department spokesman confirmed to Fairfax Media that investigators had assessed the specimens and were satisfied of their lawful origins. The department granted approval for the domestic sale of the three items only.

Mr Hill said it was only the third time since 1999 that he had seen rhino horns up for auction at Lawsons.

"I understand [the conservationists'] concerns and we have them equally. We take it very, very seriously and that's why we go through the relevant bodies to make sure we're doing the right thing," he said.

"I would not know if there is any direct correlation of the selling of antique items and increasing in poaching. If there was, I'd love to see the hard data on it."

So far this year, 969 rhinos have been poached in South Africa, according Save the Rhino. It claims poaching is "dramatically increasing".

The Western black rhino was declared extinct by the International Union for Conservation of Nature in 2011. All five remaining species are listed on its threatened species Redlist, with three classified as critically endangered.

"Elephant populations are also in big trouble in Africa and elsewhere. By continuing to sell elephant ivory, we're continuing to create demand and therefore increase poaching the populations can't sustain," Ms Wellbelove from HSI said.

On Tuesday, Greenpeace rallied its 400,000 supporters via email, urging them to contact Mr Farrah to demand he pull the horns and ivory from Friday's Natural History, Taxidermy and Science auction and change Lawsons' policies.

An International Fund for Animal Welfare report released this year revealed the number of products derived from endangered animals offered for sale on Australian websites has more than doubled since 2008.