Lead investigator Professor Elizabeth Sullivan, of the University of New South Wales, said she would expect to see similar rates of complications in New Zealand women.
In 2010, there were 749 pregnant women in New Zealand and Australia classed as extremely morbidly obese.
Auckland University Professor Lesley McCowan, an expert in maternal fetal medicine, said national data was only just starting to be collected in New Zealand.
Anecdotally, however, the problem of extremely obese mothers was growing.
"Unfortunately, it looks as if that group with extreme obesity is becoming a bit more common than it used to be,'' she told APNZ.
"It's a scary problem and it's one we've really got to start trying to address early in childhood to reverse the cycle.''
Dr McCowan said very obese women had increased risks of "virtually every pregnancy complication that you can think of''.
"There's a direct correlation between many pregnancy complications and the degree of obesity. So for example, your chance of having a stillborn baby or developing hypertensive problems or gestational diabetes - those risks increase as the maternal body mass index gets higher.''
The risks were higher not only for mothers with extreme obesity, but also those with body mass indices greater than 30.
Dr McCowan said very obese women also faced labour difficulties and were more likely to have big babies, which led to the higher rate of caesarean sections.
"It's not a straightforward delivery for them or for the doctor performing the operation, just because of the technical aspects.
"It's an increasing challenge, so operating theatres are now equipped with beds that can cope with bigger mothers and we've got different types of equipment to be able to deal with these bigger mothers when they're having their babies.''
There were also physical challenges for the staff who provided care during labour and delivery, she said.
Dr McCowan said there was a strong correlation between obesity and social deprivation.
"It's also more of a problem particularly amongst our Pacific mothers, who have higher rates of obesity in their population, and also more common amongst Maori mothers than Europeans.''
Dr McCowan said it was important that appropriate monitoring of obese pregnant women was put in place.
"One of the things we can do is to educate and help mothers to reduce their weight gain during pregnancy, and that appears to reduce the risk of a number of pregnancy complications.''
Australian researcher Dr Sullivan said she was surprised the women in the study had done "remarkably well'' despite the higher rates of complications.
There were no deaths among the women in the survey although 2 per cent were admitted to intensive care - about one in 50.
She said mothers had an increased risk of wound infections after birth, while their babies had higher rates of adverse outcomes.
