Walter was born in The Netherlands but was raised and educated in New Zealand.  He completed his medical studies at the School of Medicine, University of Auckland, graduating in 1998.  Early in his medical career he was drawn to the surgical specialties and the challenges that they offered.  In 2003 Mr Flapper commenced his advanced surgical training in Plastic and Reconstructive Surgery, being awarded a Fellowship of the Royal Australasian College of Surgeons in 2008. 

The journey continued in Australia, with a fellowship post at the Australian Craniofacial Unit (now known as Cleft and Craniofacial South Australia) in Adelaide, one of only two standalone craniofacial units in the world. A further year of training followed at the prestigious Oxford Craniofacial Unit in the United Kingdom, before Dr Flapper returned to Adelaide, where he is currently employed as a full-time consultant at Cleft and Craniofacial South Australia (formerly known as the Australian Craniofacial Unit.)

Dr Flapper firmly believes in ongoing professional development and the importance of teaching future surgeons.  In 2012, he had the honour of being the first scholar to graduate with a Master of Advanced Surgery  in craniofacial surgery from Macquarie University.  He continues to hold a teaching role at the University of Adelaide.  Dr Flapper is also involved in training future surgeons both locally in Adelaide and nationally, through courses run by the Royal Australasian College of Surgeons.   He is an examiner for the college.

Dr Flapper also serves as an International Associate Editor for the Plastic and Reconstructive Surgery Journal, one of the premier journals for Plastic and Reconstructive Surgeons worldwide.

Dr Flapper continues to work as a consultant surgeon at Cleft and Craniofacial South Australia, a Centre of Excellence in the multidisciplinary management of patients with craniofacial disorders.  His role at Cleft and Craniofacial South Australia has seen him managing patients from throughout Australia, as well as overseas.  

Dr Flapper conducts outreach clinics in Hobart, Launceston, Sydney and Berri, as well as a cleft lip and palate clinic in Alice Springs. In addition, he travels to Indonesia on a regular basis to teach local teams about the management of often challenging craniofacial conditions.

Dr Flapper consults in private practice at 226 Melbourne St, North Adelaide.