Labour leader David Shearer said commercial flights went into Myanmar very safely on a daily basis and Mr McCully could have done the same.
"We don't understand why Murray McCully has to charter his own plane, but it is part of a trend. He also took air force planes up to Vanuatu a year ago, again on the taxpayer when he could have actually taken commercial flights,'' he said.
"I think Murray McCully's being a bit extravagant given the fact that he's just cut one quarter of MFAT's staff, including people in Cairo who are helping out New Zealand citizens in really, really difficult circumstances.''
Mr Shearer said Mr McCully had his priorities wrong and if he expected staff to make sacrifices, he should do likewise.
But Mr Key today defended the decision to charter the plane, which was made by New Zealand's ambassador in Thailand, Bede Corry, rather than staff in New Zealand.
"New Zealand's a small country with a strong voice in human rights and democracy, and the options are either use a small charter plane or don't go. I'm of the view we should be going,'' he said.
"Plenty of people around New Zealand advocate very strongly that we should stand up for democracy and for human rights. That's what the foreign minister is doing. Now, I'm not going to put him in a car and put him at risk his security.''
Mr Key said he was unsure how many times Mr Corry had gone to Myanmar himself and whether he had used charter planes to do so.
"People have got to be realistic. This is the foreign minister of New Zealand going into an environment which has been, historically, certainly not one where democracy has operated, where New Zealand has been worried about human rights issues, where there have been substantial issues around home detention of the leader of the opposition,'' he said.
"This is a place where New Zealand as a small voice on the world stage can actually make a difference.''
Mfat's website said there was "high risk'' around Myanmar's borders with China, Laos and Thailand, and "some risk'' elsewhere in the country, "due to the unsettled political situation and threat from terrorism and we advise caution''.
Mr McCully's trip comes ahead of Myanmar's upcoming national elections, which follow the election last year of President Thein Sein, who is seen as a moderate.
Opposition leader Ms Suu Kyi, 66, is contesting the election as leader of the National League for Democracy, which last stood in elections in 1990.
She was detained under house arrest by Myanmar's military regime for some 15 years until her most recent release in November 2010.
