"There will be a face that people have got used to that won't be there any more," he said. "The services will continue as they always have."
Dr Stegmann said it usually took six months to find a replacement, although "more often a year".
"It's a highly specialised position. There aren't many of them wandering around in New Zealand."
They would probably have to hire someone from overseas, although they would prefer the job to go to a local.
He said hospitals such as Middlemore and Wellington would have about 15-20 obstetricians, while smaller ones like Masterton would have about two.
They would be able to find an interim replacement while looking for a permanent one, he said.
Such positions took a while to fill.
"There aren't all that many of them floating around and there aren't many floating around looking for jobs."
Dr Stegmann said it was something that happened all the time and that was how organisations worked.
They recently had two physicians and a general surgeon resign.
"We will deal with it as part of the normal ebb and flow of how things go," he said.
Wanganui's obstetrics service is part of the Regional Women's Service, which is managed by MidCentral District Health Board in collaboration with Whanganui DHB. Women's health service clinical director Digby Ngan Kee said they were positive about finding a replacement and confident there would be no change to services.