Australian bank shares rallied after Australia and New Zealand Banking Group doubled its share-buyback programme, but the overall market slipped as trade tensions between China and the United States damaged prospects for export-focussed firms.The SP/ASX 200 index fell 0.1 per cent or 6.9 points to close at 6,225.2, but still posted its best weekly performance since November 2016 with a gain of 2.2 per cent.South32 Ltd, the world's largest manganese miner, declined 3.5 per cent, while BHP slipped 0.7 per cent.Energy stocks also fell ahead of a key Opec meeting to discuss output policy later in the day.Woodside Petroleum Ltd dipped 1.5 per cent, while Santos Ltd dropped 3.4 per cent.But financials were bolstered by ANZ announcing its buyback programme would be doubled to A$3 billion ($2.22 billion).Shares of ANZ firmed 2.9 per cent to their highest since March 1, while Commonwealth Bank of Australia rose 1.3 per cent to a more than seven-week high.Across the Tasman sea, New Zealand's benchmark SP/NZX 50 index added 0.59 points to finish the session at 8,999.37 and registered a decline of 0.1 per cent for the week.Energy stocks led the gains, with Z Energy Ltd rising 2 per cent to a more than five-week high.Conversely, consumer staples accounted for most of the losses, with dairy firm a2 Milk Company Ltd dipping 1.7 per cent.