Dozens of mourners gathered at Sydney's Town Hall on Friday to pay tribute to Leak, who died a week ago of a suspected heart attack.
Editorial cartoonist for The Daily Telegraph Warren Brown opened Bill Leak's memorial service at Sydney Town Hall on Friday, paying tribute to the late cartoonist and his love of practical jokes.
Turnbull followed with a reflection on his friend of more than 30 years, delivering his speech with his portrait, painted by Leak for the 1994 Archibald prize, behind him.
"The artistic establishment couldn't accept that a newspaper cartoonist was such a talented painter," the prime minister said.
Turnbull also touched on the controversy that swirled around Leak in his final years.
"Bill Leak was accused of racism because of a cartoon, because of a cartoon," he said emphatically.
"Bill should have grown old and even more wiry."
Barry Humphries was also expected to deliver a tribute on the afternoon.
Senior Liberal figures including Peter Dutton, former prime minister John Howard and independent senator Derryn Hinch are also attending.
Outside the service, one supporter held up a poster about the late cartoonist's interaction with Section 18C of the Racial Discrimination Act.
"Wanted: for the untimely death of Bill Leak and other crimes against western civilisation, Gillian Triggs, Tim Soutphomassine, 18c and their rotten commission" it read.
Last week he praised the cartoonist for being a "good-humoured sceptic of anybody and anything in authority" throughout his "far too short life".
The prime minister is also reportedly set to fast-track efforts to change race-hate laws.
Leak was investigated for a possible breach of Section 18C over a cartoon about indigenous parental neglect, but the complaint was subsequently dropped.
Conservative figures, including federal coalition politicians, have used the case to argue the section must be changed.
Former prime minister Tony Abbott was among the guests who attended the service, speaking with several attendees including Mr Humphries.
However he avoided an awkward meeting with Mr Turnbull.
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Leak, 61, died in Gosford Hospital last Friday.
The Australian newspaper's editor-in-chief Paul Whittaker described the nine-time Walkley Award winner as "a giant in his field of cartooning and portraiture and a towering figure for more than two decades" at the paper.
