You can charm most of the people most of the time, but you can't charm a machine into co-operating with your campaign picture opportunity.

So Foreign Minister Julie Bishop discovered in Emu Plains on Wednesday, as she and local member Fiona Scott dropped in on local manufacturer RKR Engineering for a spot of talking up jobs, innovation and business tax cuts.

RKR's affable managing director Russell Ricketts was on hand to escort the two MPs around his plant, rolling out the firm's latest offering, the prototype Track Haul, a five-tonne beast designed to help cart and repair railway equipment.

Mr Ricketts invited Ms Bishop to take it for a short spin. "What could possibly go wrong?" she laughed airily.  Well, a bit as it turned out. With the petite Foreign Minister and Ms Scott at the wheel, it simply refused to move. Mr Ricketts soon solved the puzzle: "if there isn't enough weight in the seat it won't go" he declared, explaining the vehicle is designed to stop operating if a driver (presumably usually a heavier human than Ms Bishop) topples out. Fortunately for Ms Bishop, the Liberal Party is not designed along similar lines.

Ms Bishop has become a regular visitor to Western Sydney during the  campaign, with half a dozen seats hanging on slim margins. Ms Scott's seat of Lindsay is one of the most marginal (at 3 per cent) with both Malcolm Turnbull and John Howard making pilgrimages there in recent days. However, Ms Bishop declined to say if this was a sign of mounting concern in the Liberal camp. "We don't take anything for granted", she said.

Nor would she be drawn on former Abbott aide Peta Credlin's rebuke that Ms Bishop should have been better briefed on Tuesday before stumbling over the effect of the government's super changes on transition to retirement accounts. These days, she said, "Peta" is a commentator and "entitled to her view". (The Liberal camp must be getting a little weary of trotting that line out.) Ms Bishop would not concede she had given her interviewer, Melbourne radio's Neil Mitchell, a "gotcha" moment on TTR arrangements the day before, despite telling him "well, Neil, this is obviously a 'gotcha' moment". Instead, Ms Bishop was keen to demonstrate her ability to give chapter and verse on Wednesday, presumably after a thorough brush-up overnight.

Campaigning later in Blacktown, in the narrowly Labor-held electorate of Greenway, the Foreign Minister was back in her element, chatting with migrants from Syria, Afghanistan, Libya, Kenya and Sri Lanka during a short walk down Main Street.

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