SYDNEY (REUTERS) - Shares in Australian surfwear brand and takeover target Billabong International tumbled to an all-time low on Thursday on concerns about a potential takeover offer and ahead of an expected announcement from the company.

Billabong, which has been struggling with big losses, has been in talks with two takeover suitors - private equity firm Altamont Capital Partners, led by Billabong's former United States boss Paul Naude, and clothing group VF. Due diligence was expected to be completed by the end of the month.

Billabong shares were trading "on notice" on the Australian stock exchange, pending an announcement from the company. Shares in Billabong last traded down 14.2 per cent at A$0.695, having earlier sunk to an all-time low of A$0.63. Both suitors had pitched their potential offers at A$1.10 a share, valuing the company at about A$524 million (S$686 million).

"This one has got people concerned. We've seen four bids already that have all been withdrawn at the due diligence stage," said Mr Michael McCarthy, chief market strategist, CMC Markets.

"The market had more confidence around these bids because they're coming from company insiders, who the market felt would be less likely to fall away in the due diligence process but the market is obviously nervous ahead of this announcement."