** "Big Four" member Westpac WBC.AX falls as much as 3.2% to A$41.23; marks biggest intraday pct loss since March 30
** WBC among top percent losers on benchmark S&P/ASX 200 .AXJO, which is up 0.5%
** Co says net interest margin in treasury and markets division weakened amid interest-rate volatility linked to Iran war, falling to 7 bps in Q2 from 15 bps in Q1
** Notes net profit after tax will take A$75 mln ($53.2 mln) hit from costs tied to planned sale of its RAMS mortgage portfolio
** Australia's second-largest bank by assets underperforms broader financials sub-index .AXFJ, which is up 0.1%
** National Australia Bank NAB.AX, ANZ ANZ.AX down 0.2% and 0.5%, respectively
** YTD, WBC gains 8%
($1 = A$1.4100)
(Reporting by Nichiket Sunil in Bengaluru)
((Nichiket.Sunil@thomsonreuters.com))