Vietnam cancelled a long-arranged commemoration ceremony for Australian war veterans Wednesday in a move Canberra described as "a kick in the guts" that it is seeking to overturn.

More than 1,000 veterans and their families have travelled to Vietnam to mark the 50th anniversary of the Battle of Long Tan on Thursday.

"We have been working with the Vietnamese government for over 18 months for this, towards making sure that this commemoration took place in a low-key, dignified and respectful way," Veterans Affairs Minister Dan Tehan said.

"For us to be given such short notice of the cancellation is, to put it in very frank terms, a kick in the guts," he said from Canberra.

The Battle of Long Tan took place on August 18, 1966 and was the most costly single battle fought by Australian soldiers in the Vietnam War.

A force of just 108 Australian soldiers held off an assault by around 2,000 Viet Cong and North Vietnamese fighters.

Seventeen Australians were killed in action and 25 wounded, one of whom later died from his injuries while some 250 Vietnamese fighters perished.

Tehan said he was bitterly disappointed at the decision which he said "should not have occurred".

"My hope is that the Vietnamese government will overturn it," he said, adding that Prime Minister Malcolm Turnbull was hoping to speak to his Vietnamese counterpart on the issue.

As commemorations marking Long Tan began in Australia, Tehan said Canberra had gone out of its way to ensure the Vietnam event was low-key.

"We understand that there are sensitivities still in Vietnam," he told reporters.

- Police turn people back -

Tehan said it appeared the decision was based simply on the sensitivities relating to Long Tan, and did not relate to any other aspect of the relationship between Canberra and Hanoi.

Australia's Department of Foreign Affairs said while the Vietnamese authorities had cancelled the commemoration ceremony for Vietnam Veteran's Day at the Long Tan Cross site on August 18, private access to the site may still be permitted.

Vietnamese authorities in both Hanoi and southern Ba Ria Vung Tau province, where the battle took place, did not respond to requests for comment.

Liam Cochrane, an Austrlaian reporter with ABC covering the anniversary, said Vietnamese police were stopping people accessing a memorial cross on the battlefield on Wednesday.

Australian veteran Peter Taylor, who served in Vietnam in 1969-70 after the Battle of Long Tan and who organises battlefield tours in the region, said the decision came as a shock.

"We've only just found out," he told AFP by phone. "I just can't believe it."

"We do commemorations every year. There are about a thousand Australians who have come out for this one. There's never been a problem before so we're not sure what has happened."

He said there was a planned gala dinner on Thursday night for about 600 people, including up to 80 Vietnamese veterans of the battle.

"It's not just the Aussies, there are lots of Vietnamese who come out for these ceremonies. We have a very good relationship with them."