The Turnbull government was on Wednesday night seeking an urgent change of heart by Vietnam after the country delivered a "kick in the guts" by banning Thursday's 50-year Long Tan battle commemoration.

The abrupt decision by Hanoi to stop the commemoration service – which was 18 months in the planning – because of lingering sensitivities about the war has left in limbo about 1000 veterans and their families who have already travelled to Vietnam.

Late on Wednesday, the Department of Foreign Affairs was advising Australians not to travel to the site east of Ho Chi Minh City, saying "we understand that access to the site is currently blocked" and this might continue on Thursday.

A visibly upset Veterans Affairs Minister Dan Tehan said Foreign Minister Julie Bishop expected to speak with her counterpart on Wednesday evening and Prime Minister Malcolm Turnbull was also seeking a conversation with Vietnam's leader.

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

"This is a decision which in my view should not have occurred and my hope is that the Vietnamese government will overturn it."

The Australian government had been told of the cancellation only on Tuesday afternoon, Mr Tehan said.

Furious veteran Ken Aspinall, from Tamworth in NSW, told Fairfax Media from Vietnam that that some veterans may still try to go to the site, where a white cross stands above farmland.

"I should have brought my bloody gun over … Bugger them. We came all this way," said Mr Aspinall, 68, who was wounded and almost died in the Vietnam war but did not fight in the Long Tan battle.

"We'll see what happens tomorrow … maybe we'll walk right over them," he said.

John Bell, the New Zealand owner of Belly's Watering Hotel bar in nearby Vung Tau, said the announcement had shocked his customers who were there for the commemoration.

"It's bullshit. People have packed into this joint. We have buses lined up to take the people out to the cross tomorrow," he said. "While it's disappointing it's not surprising. The Vietnamese are a funny lot … they might change their mind and it might go ahead at the last minute. We can only hope."

Mr Tehan said Australia had been working with the Vietnamese government for more than 18 months on the planned service and understood the sensitivities about the war. The planned service was "low-key, dignified and respectful", he said.

Seventeen Australians were killed at the battle of Long Tan in 1966. A further 25 were wounded, one of whom later died from his injuries.

Mr Tehan said he understood that an official party including the Australian and New Zealand ambassadors would be able to lay wreaths at the site and small groups could also visit.

Vietnam Veterans Association of Australia president Ken Foster said he even feared for mental welfare of some of the former soldiers.

"I do have concerns for the mental welfare of not only those Vietnam veterans in Vietnam but also those here in Australia. When they hear this news...there's going to be some fallout from it."

He said Australian forces were well-known for the respect they paid to the Vietnamese war dead.

"All our people wanted to do on this occasion was to go back there and pay their respects to our war dead on the site where they died ... I would not expect the veterans to react in any other way apart from their own personal deep sadness but they will be affected by this."

The battle has remained a sensitive issue for the communist country's rulers for 50 years.

Vietnam banned Australians from wearing medals or uniforms, or carrying flags, at a dawn service held at Long Tan on Anzac Day.

Vietnam propaganda claimed that the battle was won by its soldiers, claiming it had wiped out "almost a complete battalion (about 800 men) of Australian mercenaries"

The battle site, which is now a farm, was open to visitors earlier this week.

But an ABC journalist was stopped by authorities 200 metres away on Tuesday. No reason was given.

Australian veterans have organised a gala dinner for Thursday evening at a hotel in the Vietnamese town of Vung Tau that is expected to be attended about 40 Vietnamese veterans.

The former enemies met in a series of ceremonies in 2015.

Australian veterans, including Harry Smith, who commanded soldiers at the battle, will be attending the dinner organised by Australian businesspeople in Vung Tau.

Money raised will go to Vietnamese charities. 

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