The date has not been formally revealed however it is believed to be at midnight tonight [overnight New Zealand time].
Chan married his Indonesian fiance on Monday on Nusakambangan island in a ceremony conducted by Salvation Army minister Dave Soper and witnessed by close family and friends.
Sukumaran has spent his final hours painting harrowing self-portraits.
Mr McCully said that another formal approach from New Zealand was unlikely before tonight.
"This is the sort of space where the countries that are directly concerned with citizens involved need to be given an opportunity to do their work without others interfering.
"We have made clear our views on capital punishment generally, we have expressed our concern about the individuals involved. I have had an extensive conversation with the Indonesian Foreign Minister about it."
The Australians' impending execution comes as Wanganui man Antony de Malmanche fights drug charges that could lead to a death sentence in Bali.
Labour's acting leader Annette King said waiting until after the executions of Chan and Sukumaran went ahead was pointless and New Zealand should make its views clear before then.
"It's a bit late after the executions have taken place. If he is going to make any impact at all it would be wise for him to say it before the executions."
Andrew Chan is lead from a court holding cell. Photo / Getty Images
She said De Malmanche's case added to the need to speak out now.
"I don't think it would do any harm at all for New Zealand to join with Australia and express our opposition to the death penalty. We are opposed to it. But the truth is, at the end of the day a sovereign country will do what it will."
Mr McCully said the De Malmanche case was "clearly a consideration in how we deal with the circumstances involving the Australians".
Asked about whether it was appropriate to pursue trade and other measures with countries where capital punishment takes place, he said such engagement was a factor in improving human rights records around the world.
Prime Minister John Key will visit Saudi Arabia tomorrow on a trade mission. The Green Party have said a trade deal is inappropriate given the country's continued human rights abuses including beheadings.
