Alison Johns, 42, of Barnet, north London, is a neo-natal nurse at University College Hospital, London. She is furious about the condition because she says she has an excellent work record and no health problems.
Ms Johns was born in Britain but brought up in Melbourne from the age of 12, when her father emigrated there. She returned to Britain at 21 and later married. After a divorce two years ago she decided to rejoin her family.
She was granted a court order last year to take her children, Peter, 10, and Emma, 13, abroad, and told by the Australian High Commission she would get preferential consideration because she had close relatives in the country. But after a compulsory medical examination Ms Johns, who is 5ft 5in, was amazed to learn she would have to lose five and a half stone before her application would be approved.
She said: 'I was flabbergasted. Although they didn't give me a time limit, they said nothing would happen until I had done it.' The letter warned: 'Failure to comply exactly will probably cause delay in processing your application.'
Ms Johns has attended Weight Watchers since February and kept to a diet which cuts carbohydrate and emphasises fish and vegetables. She is now 14st 2lb.
Yesterday Susie Orbach, author of Fat is a Feminist Issue, said: 'This policy is outrageous. How many anorexics and bulimics who throw up in their toilet bowls every morning are they letting in? Being underweight can be as much of a health hazard as being overweight.'
Others disagreed. Professor John Garrow, who advises on human nutrition at St Bartholomew's Hospital, London, said: 'At 18st 6lb this lady was severely obese and very likely to require health care in Australia. If I was running their health services I would rather she arrived weighing 13st.'
An Australian High Commission spokesman refused to comment on the case, but said obese applicants were asked to lose weight because they were less likely to make demands on the free health care service provided in Australia, a system which had little spare capacity.