South Africa will not allow shale gas drilling in the vast central Karoo to threaten its bid to host the world's largest radio telescope, the science ministry said on Tuesday.

South Africa is competing with Australia to win the contract for the Square Kilometre Array (SKA), a multi-billion dollar instrument that will be 50 times more sensitive than today's most powerful radio telescopes.

"We will ensure that none of our environmental considerations, water, resource considerations, nor our astronomy endeavours will be jeopardised by proposed hydraulic fracturing," said deputy minister Derek Hanekom.

The state hopes to build the telescope in the vast Karoo where underground gas deposits are being eyed for controversial hydraulic blasting by several mining companies, including Anglo-Dutch giant Shell.

"We remain very positive and hopeful that we will emerge victorious when the winning bid is announced in 2012," said minister Naledi Pandor.

The minerals ministry recently halted all new applications and any decisions to explore for gas in the Karoo while it carries out a study after a massive public backlash against the "fracking" process over environmental fears.

The science ministry has legal authority to protect the region from light or radio interference which could affect the SKA, planned for a remote, quiet Karoo corner, she said.

"Clearly if any project or activity were to be a threat to our astronomy facilities, we would use the act," she told AFP.

Scientists hope the SKA, which links 3,000 antenna dishes, will shed new light on fundamental questions about the universe, including how it began, why it's expanding and whether it contains life beyond our planet.

Australia and South Africa are the last two candidates in a five-year competition that has seen an international steering committee narrow the field from five proposals to two.

The final bid will be submitted later this year.