Two rare black rhinoceros at Taronga Western Plains Zoo are pregnant and expected to give birth next year.

The exciting development prompted a visit from NSW environment minister Rob Stokes, who described the zoo's breeding program as "critically important" given the threats the species faced around the world.

"In their homelands in Africa we have a shocking situation where rhinos are poached for their horns which are used in traditional Chinese medicine or are used in ceremonial daggers," he said.

"They are hunted and under critical threat, we've already seen one sub-species declared extinct just three years ago, so the program here at Taronga Western Plains Zoo is absolutely critical at a global level.

"In 2013 over 1000 rhinoceros were killed in South Africa alone, the worst year in the past decade.

"The appetite for rhinoceros horn isn't slowing down with this year almost 500 rhinoceros poached in South Africa already, every birth is critically important for the species."

The pregnancies also were testament to the wonderful work done by the dedicated zoo staff, Mr Stokes said.

"The zoo has been very successful in breeding black rhinoceros across the program's 20-year history, producing 11 black rhinoceros calves to date."

The zoo's general manager Matt Fuller said what was especially exciting was that two rhinos were pregnant at the same time, the first time it had happened at the zoo.

With a gestation period lasting between 14 to 16 months, mothers Bakhita and Kalungwizi would hopefully give birth to their healthy calves in mid-2015, he said.

The conceptions were the result of "natural mating" rather than insemination, Mr Fuller explained.

"Kalungwizi has had a number of calves here at the zoo over the years, including Bakhita and this is Bakhita's second calf," he said.

"In her case it will be a third generation animal at the zoo, it's a really big milestone in the history of the program."

Where the calves would live when they were older would depend on how the breeding program progressed in the next couple of years, Mr Fuller said.

The program could see the young rhinos stay in Dubbo, sent interstate or overseas, but what was certain, Mr Fuller said, was they would play a critical role in both restocking and understanding black rhinos.

"Taronga Western Plains Zoo is the only zoo in Australasia that breeds the black rhino, we are part of an international program coordinated in the US and we have been the most successful breeder of the species outside the African continent," he said.

Mr Fuller was confident the zoo was equipped to deal with any 'morning sickness' or cravings that might ensue.

"As a zoo we have to handle lots of unique situations so we are well prepared, whether they have cravings for casuarina, lucerne hay or acacia, we'll be onto it."

Mr Stokes, who fed casuarina and kurrajong branches to expectant mother Bakhita during yesterday's visit, said it was not his first visit to the zoo in Dubbo.

"I came here as a child and recently again but there have been a lot of changes, it's absolutely magnificent, and I'm really looking forward to having a proper look and camping here tonight."