She was finally freed last year and Burma's military regime has since released 650 other political prisoners, legalised trade unions and relaxed media censorship.
It has invited 159 foreign observers to monitor tomorrow's polls.
Mr McCully, who met Ms Suu Kyi in Rangoon on March 7, said he had arranged to ring her again a few days after the byelections to see whether she felt they were free and fair.
New Zealand would be guided by her view, as well as its own observers, before deciding whether to support easing trade sanctions.
He said the decision on sanctions would be "a matter for international discussion", but there was agreement on a "checklist" including free and fair elections, release of political prisoners and moves towards settling the ethnic conflicts.
"All the signs are that the regime is going to almost challenge the international community to review the sanctions at an early time, and I think that ... that is something that is going to happen," he said.
"I wouldn't be surprised to see quite significant moves sooner rather than later."