Vietnamese abattoir workers have been secretly filmed bludgeoning Australian cattle with sledgehammers, animal rights activists say.

Graphic footage released by Animals Australia shows a cow being clubbed over the head five times.

The man, said to be a Vietnamese slaughterhouse worker, continues to hit the cow on the ground before it dies.

The government has said it is investigating but Agriculture Minister Barnaby Joyce ruled out an export ban.

The latest footage has been released a year after officials promised to investigate similar allegations about the treatment of Australian cattle in Vietnam.

Australia exported 227,563 cows to Vietnam in the 12 months to the end of April, according to the Australian Livestock Exporters' Council (Alec).

Animals Australia said just two of 13 slaughterhouses it visited in Vietnam met Australian requirements.

It also alleged that Australian cattle had been found in 11 non-accredited abattoirs, indicating the failure of traceability systems.

"Nobody can claim they didn't know this was happening - the exporters knew, the department knew and the minister knew," Lyn White of Animals Australia said.

Alec said it would suspend exports to the abattoirs under investigation.

'Inexcusable'

It also said it would review its systems that oversee control and traceability, which would include ensuring that CCTV systems were working at abattoirs receiving Australian cattle.

Animals Australia exposed similar practices in Vietnam last year but says Alec failed to respond to its findings. The government said at the time it would investigate.

Alec boss Simon Crean said the latest footage was "inexcusable", the ABC reported.

Australia's live cattle export industry is estimated to be worth about A$800m (£415m) a year.

There have been numerous controversies over the treatment of cattle abroad.

In 2011, footage of cruel treatment in Indonesian abattoirs prompted the government to ban live cattle exports to that country, while exports to Egypt were suspended in similar circumstances in 2013.