Australia's deepest offshore drilling operation has won approval from regulators before its environment plan has been made public under disclosure rules that have since been tightened.

Esso this week won approval from the National Offshore Petroleum Safety and Environmental Management Authority for plans to drill for gas about 100 kilometres off the Victorian coast in Bass Strait in its Sculpin prospect.

Drilling deeper: an oil rig in the Bass Strait operated by Exxon's Australian subsidiary, Esso. Credit:James Davies

The company, owned by US energy giant Exxon, expects to drill as deep as about 2300 metres, or slightly deeper than the 2240-metre oil well proposed by Norway's Equinor in the Great Australian Bight, the agency said.

New rules to increase transparency of environmental plans for offshore drilling came into effect on April 25 - but don't apply to either Exxon or Equinor as their proposals were lodged earlier, an agency spokesman said.