Luckily, South Australia does not grow balsa trees, or we might just have committed $50 billion to submarines that won't submerge. Instead, they'll be built from high-quality non-buoyant Australian steel.

Such was Malcolm Turnbull's determination to reverse the Abbott government's indifference to the death of the automotive sector, and to avoid the perception of any similar indifference to a decline of the state's remaining defence materiel industries.

And such is the power of a relentless Nick Xenophon and an uncompromisingly parochial Weatherill Labor government, both of whom have ratcheted up the fear of defeat for Christopher Pyne - the state's most senior, and doubtless most annoying member of the Turnbull cabinet.

Within conservative circles, there has long been a view that if the SA situation could be fixed, then the Coalition would be most of the way towards securing the federal election - or at least silencing the indefatigable Pyne.

Which is pretty amazing when you consider how few seats are really in play.

But then, as one wag quipped on Twitter after Turnbull's notably Churchillian delivery of the good news for SA: "Never before has so much been spent to the political benefit of so few".

That "so much" is big alright, representing a mega-win for Pyne and his nervous South Australian colleagues. At an eye-watering $50 billion, the construction of all 12 future submarines fully in Adelaide, is a huge boost for a small state. And the fact that the subs will be constructed using local steel will achieve crucial political purchase also, especially given the problems of Arrium and the sense that Canberra was again sitting on its hands.

And the "so few"? Pyne's own eastern-suburbs seat of Sturt is now safe, but with a margin of over 10 per cent, it would have been amazing were it otherwise. So too Mayo, held by the recently benched minister Jamie Briggs, which has an even higher margin of 12.5 per cent. The other two lower house seats in doubt, Boothby (7.5 per cent) and Hindmarsh (1.9 per cent) are far less certain although both will be helped by this decision.

That Briggs and Pyne are so worried, speaks to the economic insecurities in SA's narrowly based provincial economy. And it suggests the Liberal Party's own research showed an annihilation was on the cards in the central state.

That in turn could have unleashed all kinds of dysfunction for Turnbull, perhaps even including defections.

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