Don't risk Wentworth protest vote: Howard

It's been reported senior Liberals wanted Malcolm Turnbull to come out strongly in support of Dave Sharma but instead Wentworth voters heard from John Howard.

Former prime minister John Howard has told Liberal voters in Wentworth if they don't vote for the party's candidate in Saturday's by-election they risk "robbing" the Morrison government of its parliamentary majority.

"If enough people decide to register a protest vote then the seat could go to an independent and that will rob the Morrison government of its working majority and make the remaining months of this term of office very difficult," the former Liberal leader said when campaigning alongside candidate Dave Sharma in Double Bay.

It's been reported senior Liberals wanted former Wentworth MP and ex-PM Malcolm Turnbull to come out strongly in support of Mr Sharma but, in his absence, the party on Thursday brought out Mr Howard.

"There's no such thing in modern politics as an unlosable seat," he told reporters.

"This seat may have been held by the Liberal Party or its predecessors since federation but every election and every by-election is different."

Mr Howard acknowledged people were "grumpy" about the Liberal Party knifing Mr Turnbull as leader in August.

"A lot of people here are sad that he's not any longer the member and prime minister but others will take a view projecting forward," he said.

"They'll say we're unhappy about that but we're a lot more unhappy about the thought of having a Shorten (Labor) government snaffling our hard-earned savings."

Mr Howard joined Mr Sharma and Prime Minister Scott Morrison in condemning an email which falsely claimed independent frontrunner Kerryn Phelps was withdrawing from the contest because she'd been diagnosed with HIV.

"I think it's disgusting and I utterly repudiate it. It deserves our contempt," he said.

Mr Sharma in Double Bay said he'd welcome a police or electoral commission investigation into who was behind the "vile" email.

"These sort of things are despicable and they should be confronted and combated," he said.