thisThis format is working for me
$csvDate = [datetime]::ParseExact("23/09/2015 05:41:27","dd/MM/yyyy HH:mm:ss",[Globalization.cultureinfo]::GetCultureInfo("en-US"))
$currentItem["Publish Date"] = [Sitecore.DateUtil]::ToIsoDate($csvDate)
$csvDate = [datetime]::ParseExact("23/09/2015 05:41:27","dd/MM/yyyy HH:mm:ss",[Globalization.cultureinfo]::GetCultureInfo("en-US"))
$currentItem["Publish Date"] = [Sitecore.DateUtil]::ToIsoDate($csvDate)