[1] Customers of NAB (National Australia Bank) were not able to access online banking last January 6th, 2015.
[2] It is because their online banking access was affected by the high of temperature in Perth, Australia.
[3] NAB spokesman said that they post notifications and updates through posted Facebook page and Twitter account.
Source: http://www.itwire.com/business-it-news/business-technology/66586-nabs-online-banking-site-goes-kaput