According to a Bloomberg news report, the Woodside-led NWS Venture has halted output at the Cossack-Pioneer production vessel, while Santos has shut-in its Mutineer-Exeter oil field.

A category one storm, Tropical Cyclone Isobel was at 1pm (WST) crossing the coast of northwestern Australia about 110km northeast of Port Hedland packing winds of about 100km/hr, the Bureau of Meteorology said on its website.

But as Isobel is a "very weak system," the bureau expected it would weaken into a tropical low several hours after reaching land.

"Conditions should ease significantly as the system moves inland and weakens," it said.

"Heavy rainfall may continue in the western Kimberley and adjacent parts this afternoon, easing tonight."

The Cossack-Pioneer produced about 106,000 barrels a day of oil in the September 2006 quarter, while Santos’ Mutineer-Exeter field was producing 55,000bpd in late October.

The 2006-07 cyclone season was not expected to be as active as the previous one, the bureau said in October. It forecast one or two tropical cyclones were likely to affect the coast this season, with the total number being fewer than the average of five.

Last season saw four cyclones crossing the northwestern coast and two others affecting the coastline. Woodside Petroleum, Santos, BHP Billiton and Tap Oil were among petroleum producers that were forced to shut fields due to the storms.