I doubt that the english tank stems from the portugese word. :-( AFAIK, tank was a codename for WWI tanks (cf. http://en2.wikipedia.org/wiki/Tank#History). I have no idea where the original tank stems from...
Australian & Indian sense "dam"
As a middle-aged Aussie I've never heard this usage in Australia. Though there was a feature in early Sydney called the "tank stream" and a lane is still named after it. I don't know how to mark just the Aus. part as archaic, obsolete, or historic. In India it's still normal modern use there though. — hippietrail (talk) 03:22, 13 April 2015 (UTC)