Australia kicked over a last-gasp penalty to edge hosts Argentina 20-19 in the Rugby Championship on Saturday, overcoming appalling weather to win their first match of this year's competition.Substitute Ben Donaldson knocked over his effort from close range with the final action of the match to see the Wallabies to narrow victory in cold and rainy conditions at Estadio Jorge Luis Hirschi in La Plata.Australia outscored their hosts two tries to one with Jake Gordon and Rob Valetini both going over and Noah Lolesio adding a penalty and two conversions.Argentina's try was scored early by loose forward Juan Martin Gonzalez, while Santiago Carreras contributed 11 points with three penalties and a conversion.The victory will be a major fillip for the Wallabies after being comprehensively beaten at home by South Africa in their opening two matches of this year's Southern Hemisphere championship, although they remain bottom of the standings at the halfway mark."(I'm) really proud," said Australia head coach Joe Schmidt. "We have a lot of work to do but we're pleased to get the four points and we're in the contest."The test in La Plata was always going to be a scrappy affair given the foul weather but it also saw many positives, mainly on defence, for both teams.After seeing off more than 30 phases of attack from Australia after the start, it was Argentina who opened the scoring with a Carreras penalty in the 12th minute followed three minutes later by a pushover try from a lineout maul for GonzalezArgentina's defence were busier than their attack in the first half, however, and they held out under persistent attack from the visitors until the Wallabies eventually managed to prise them open with a 27th-minute try from scrumhalf