Beach Energy has shocked investors with a big downgrade in reserves at a nearshore field in Victoria that started up only two months ago, fuelling concerns that years of underperformance at the Kerry Stokes-backed oil and gas producer have not ended.

Shares in Beach, which counts the billionaire’s Seven Group as its biggest shareholder, slumped 12.6 per cent on Monday after its chief executive, Brett Woods, said he was “extremely disappointed” to downgrade profit expectations, especially after having reported other reserve revisions across the portfolio in June.