Robben Island authorities are to embark on a major cull of the island's rabbits, fallow deer and cats next month.

"The island is heading for... an environmental disaster," the island museum's acting CEO Jatti Bredekamp said on Friday.

He said the 25 000-plus rabbits on the 475 hectare island were causing "absolute havoc", destroying vegetation and burrowing under historic buildings.

The animals would be shot by a team of experienced professionals, working after the last tourist of the day had left, and in the early mornings.

At the same time, the department of public works would help eliminate the island's rats.

Island officials said the rabbits and the around 500 deer had stripped virtually all the edible vegetation from the island.

"The rabbits have actually started eating stinging nettle," said nature conservator Estelle Esterhuizen.

"Right now we've got to do a drastic drop in numbers. We've got to. The veld just can't handle it."

She said rabbits had even been seen climbing invasive rooikrans bushes to eat the leaves.

Esterhuizen said there were about two dozen cats on the island, and that instead of preying on the island's rats, they went for easy targets -the chicks of penguins, the swift tern and Hartlaub's gull, of the threatened oystercatcher, and of the highly endangered bank cormorant.

Officials said with the stripping of vegetation, windblown sand was making roads impassable and in one spot had created a three-metre drift against a harbour wall.

It also hampered attempts to eradicate the invasive rooikrans, as it could not be chopped out unless something was able to grow in its place to hold the sandy soil.

And penguin burrows, used for nesting, were also collapsing because there was no root structure to hold the soil above them.

Environmental officer Mario Leshoro said a census earlier this year counted only 2400 breeding penguin pairs on the island, compared to more than 6000 in 2007.

Rabbits were brought to the island by early sailors, to breed as a source of meat.

The fallow deer, which come originally from Europe, were introduced in the mid-20th century.

There have been what Esterhuizen described as "small" culls of the deer in the past, to keep their numbers down, as well as periodic culling of the cats.

She said a euthanasia programme had been introduced for the rabbits in November last year, involving trapping followed by lethal injection.

However after initial successes, the creatures had become trap-shy, until only five to eight animals were being caught every night.

This had not been enough to keep up with the rate at which they reproduced.

Acting heritage resources and environment manager James Makola said the corpses of the culled animals would be buried on the island.

The cull would continue for four months, following which authorities would assess the situation and look at a management plan for the next three to four years.

He confirmed however that the island's ultimate goal was to get rid of all its rabbits, deer and cats.

There is a small herd of springbok on the island, as well as some steenbok.