Mr Howard, who maintains all efforts to save Nguyen have been exhausted, said he had an obligation as host to attend the Prime Minister's XI cricket match on Friday.
Nguyen will hang in Singapore's Changi prison at dawn on Friday after he admitted smuggling heroin to repay debts owed by his twin brother Khoa.
The Opposition Labor and the Democrat parties have demanded the cricket match be called off out of respect for Nguyen.
Labor senator George Campbell said Mr Howard's justification for attending the game was feeble.
Protest call
"I think it's an outrage that the match should go ahead and if he has any support for the abolition of hanging then he wouldn't go to the match on Friday," Senator Campbell told reporters in Canberra.
"He would lead a protest in Parliament House against it. Of course call the match off, of course he's being insensitive."
Australian Democrats senator Natasha Stott Despoja agreed, saying she felt sickened by the prospect that Mr Howard would attend the game.
"This is about how Australians and the rest of the world, including the people of Singapore, will view our response to this horrendous act - if indeed it goes ahead," she said.
"The prime minister has to show some gravitas and at this late stage make it very clear that the last thing he will be doing this week is playing cricket."
But Mr Howard has insisted he is obliged to attend, and he hoped Australians would understand.
"I have a duty as the host to go to that match," Mr Howard told ABC radio.
"I think the Australian people will understand that I didn't set the date of this man's execution. I wish there was no date set for his execution."
Nguyen's mother Kim, brother Khoa and two of his closest friends Bronwyn Lew and Kelly Ng are in Singapore to say their final goodbyes this week.
Nguyen will not be allowed visitors before he is hanged in the early hours of Friday morning.
Hopes downplayed
Australia’s Attorney-General Philip Ruddock has downplayed new hopes held by Nguyen's legal team that the execution could still be stopped by taking the case to the International Court of Justice.
He said Singapore had repeatedly said it would not accept the jurisdiction of the court in the matter.
Singapore's Prime Minister Lee Hsien Loong told Mr Howard that just when Mr Howard made a fifth and final plea for clemency during the Commonwealth Heads of Government meeting in Malta.
Nguyen's lawyer Julian McMahon is refusing to give up hope, again calling on Australia to invite Singapore to test its mandatory death penalty in the court.
"We have a dispute between the law of the first world, which says a mandatory death penalty is wrong, and Singapore's law where they say, no that's okay," said Mr McMahon.
"Singapore then says its system is open and fair, well we want to challenge that ... we say that's a genuine legal dispute."
Mr Howard has ruled out using economic sanctions in a last ditch bid to persuade Singapore not to go ahead with the execution, but the ACTU has said the unions will join in if there's a call for trade sanctions against Singapore, but won’t take unilateral action.
Yellow ribbon
Mr Ruddock said the government may consider a request for a minute's silence to observe Nguyen's hanging.
Meanwhile, Australians are being urged to wear yellow ribbons as a gesture of support for the condemned man.
Nguyen was arrested in Singapore's Changi Airport in December 2002 while trying to board a flight to Australia with 396 grams of heroin strapped to his body and in his hand luggage.
He was sentenced to death despite cooperating with drug investigations by authorities in Singapore and Australia.