Shell has joined the hunt for oil in the Great South Basin and is ready for any protest action that may target new exploration.

The operator of the Tui offshore Taranaki oil and gas field says probable reserves are between nine and 11 million barrels of oil short of the 50.5 million barrels previously thought.

Budget documents reveal a power cable to the Kupe gas and oil field has failed and is the subject of a $95 million insurance claim.