There are concerns for the mental health of Long Tan veterans who have returned to the site of their famous battle, after a last-minute decision by the Vietnamese Government to cancel today's 50th anniversary commemorations.

Veterans' Affairs Minister Dan Tehan said he was bitterly disappointed with Vietnam's decision to cancel the commemorations at Long Tan.

Sorry, this video has expired Dan Tehan says decision to cancel a "kick in the guts"

Mr Tehan said the Vietnamese Government notified Australia of its decision late on Tuesday, citing deep sensitivities in the country.

He told reporters in Canberra this afternoon that more than 1,000 Australians had travelled to Vietnam to mark the 50th anniversary of the battle and he was hoping the Government reversed its decision before today.

"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.

"I don't think this is the way that you should treat a friend.

Prime Minister Malcolm Turnbull has requested a phone call with his Vietnamese counterpart over the decision.

Mr Turnbull said he was deeply disappointed over the decision.

"We understand it is a matter for the Government of Vietnam to decide what commemorations are held in its country," he said.

"But this decision, and especially its timing, shows a disregard for those Australians who have in good faith travelled to Vietnam to participate in this week's events."

A Vietnamese Government source told the ABC the Australian Consulate promised to hold a "low key" event.

However, the gala dinner, concert and the expectation of more than 1,000 Australians at the Long Tan memorial cross was seen as an insensitive celebration.

"The Vietnamese Government has advised the Australian Government that it will not permit the ceremony to commemorate Vietnam Veteran's Day at the Long Tan cross site at 3:30pm on August 18 to proceed," DFAT said in a statement.

"An official party, including the Australian and New Zealand ambassadors, will lay a wreath and we understand small groups will still have access to the site on the day.

"The Government is deeply disappointed at this decision, and the manner in which it was taken, so close to the commemoration service taking place.

The news came after Vietnamese police blocked access to the Long Tan memorial site, as local officials discussed whether to allow a planned event to go ahead today.

West Australian Vietnam veteran Peter Manucci and his tour group of "Vietnam gunners" were refused entry to the battlefield on Wednesday.

"I am pretty well pissed off," he said.

"There is 3,500 people who are booked in to go the Long Tan cross and that's why they are not happy."

Mr Manucci said each visitor paid around $12 in advance to see the site.

"They have our money ... that's $30,000 in their pocket, gone, thank you very much," he said.

An ABC crew was stopped 200 metres in front of the memorial cross. ( ABC News: Liam Cochrane )

An ABC crew was stopped about 200 metres before the site's memorial cross.

No clear reason was given for blocking access to the site, which is on a private farm, but access to the cross was open on Tuesday.

The anniversary of the Battle of Long Tan has long been a delicate topic in Vietnam.

Each year small groups are allowed to visit the cross, but no medals, uniforms or songs are allowed.

Since 2006, the Vietnamese authorities have allowed the playing of The Last Post.

Memorial events have been held at Long Tan since 1989.

For many veterans, this would have been their first visit to Vietnam since the war.