Former prime minister John Howard has urged voters to use the Canning by-election to send Opposition Leader Bill Shorten a message to stop obstructing the free trade agreement (FTA) with China.

Mr Howard also denied he had appeared with Liberal candidate Andrew Hastie in the electorate yesterday because the campaign was struggling, saying Mr Hastie was so impressive "that I wanted to be part of the action".

Mr Howard and Mr Hastie walked through a Mandurah shopping centre where the former prime minister was mainly warmly welcomed, shaking hands and taking photos with shoppers.

Federal Labor has refused to guarantee support for the FTA, with the Opposition and unions saying Chinese people could be given work in Australia ahead of local workers.

Mr Howard said current and former Labor leaders were "falling over themselves" to condemn Mr Shorten's position on the free trade agreement.

Former Prime Minister John Howard and Andrew Hastie with school children in Mandurah. ( ABC News: Nicolas Perpitch )

"The very idea that Bill Shorten would stand in the way of this free trade agreement demonstrates how hog-tied he is to the union movement, and particularly the militant [Construction, Forestry, Mining and Energy Union]," Mr Howard told reporters outside the shopping centre.

Mr Shorten, who was also in Canning yesterday, in the suburb of Armadale, said people had come up to him expressing their concerns with the China free trade agreement.

"They can't understand why [Prime Minister Tony] Abbott hasn't chosen to stand up more strongly for Australian jobs," Mr Shorten said.

The Opposition Leader said Labor wanted to change the enabling legislation around the FTA, and it did not accept that for projects over $150 million there should be no requirement to have labour market testing.

"I think that when you've got 90,000 plus unemployed Western Australians, having an agreement which removes any safety net for labour market testing for projects over $150 million, it's a dud deal," he said.

Mr Howard rejected a question suggesting he had come to Canning because the Liberal campaign was struggling.

"It just shows I'm a compulsive campaigner and I happen to be coming over to Western Australia and I couldn't resist offering myself to help," Mr Howard said.

"He's such an impressive candidate that I wanted to be sort of part of the action.

"It's got nothing to do with desperation."