Hospital ship charity Mercy Ships passed a significant milestone on Monday: a team aboard its flagship Africa Mercy carried out the Christian organisation's 100,000th free surgical procedure.

Docked for a 10 month stay in the west African nation of Guinea, the milestone was passed when Australian plastic surgeon David Chong performed a cleft lip correction on seven-month-old Aissata.

Dr David Chong and Aissata

Dr David Chong with Aissata after her cleft lip surgery. PICTURE: Mercy Ships.

Chong, who is president of the Australasian Cleft Lip and Palate Association and who volunteers with Mercy Ships twice a year, described his involvement in the passing of the milestone as "humbing".

“[E]specially when you think about the incredible legacy that Mercy Ships has built over the past 40 years by healing thousands of patients like Aissata,” he said. “So many other volunteers have helped change lives for the better, and I’m proud to contribute to this powerful work.”

The organisation, which relies on volunteers, is this year marking the 40th anniversary of its founding.

Speaking of the passing of the 100,000th procedure, American founder Don Stephens said he was "excited to reach this significant milestone and honoured to be a part of transforming so many lives".

As well as performing 100,000 surgical procedures during its 40 year history, Mercy Ships - which has worked in 56 countries - has trained more than 42,000 local professionals including surgeons, anaesthetists and midwives, delivered more than $A1.5 billion of services, and completed more than 1,100 agricultural and infrastructure development projects including building and renovating medical and dental clinics.

The Africa Mercy will leave Guinea in early June and sail for Senegal in August.