Hostages at the Lindt Cafe hold the Black flag.YouTube 
A member loyal to ISIS waves its flag in Raqqa on June 29, 2014. Initial reports claimed that the flag used in the Lindt Cafe was an ISIS flag.Reuters 
Some reports claimed that the Lindt cafe flag then claimed that the flag belonged the Al Qaeda splinter group is Syria, al Nusra,Twitter 
Saudi Arabia national flag.Reuters 
The flag used by the Nigerian Sunni Terror group Boko Haram.Twitter
Apparently, the 'black flag' raised by the hostages in Lindt Cafe belongs neither to Islamic State (ISIS) nor to the Al Qaeda splinter group in Syria -- Jabhat al Nusra. It is most likely to be just an Islamic flag.
The black Islamic flag carries writings in Arabic, which reads, "There is no God but Allah, and Muhammad is his messenger". This underlines the basic belief of Islam.
When the hostage story was still developing, several reports suggested that the 'black flag' at the cafe belonged to ISIS. A Daily Mail report then claimed that the flag used in Sydney cafe siege is used by Syrian extremist group Jabhat al-Nusra.
However, it has to be noted that it is a Shahada flag and is used to by most Muslims to bear witness that Allah is the only true God, and Muhammad is Allah's prophet.
The flag thus shares similarities not only with an ISIS flag, but comes 'very close' to the banner used by the Jabhat Al-Nusra (the Nusra Front), the Islamic group fighting against Bashar Al-Assad in Syria. A similar flag is also used by Boko Haram in Nigeria. Besides, Saudi Arabia's national flag, which is green in colour too carries the same message.
According to a senior news editor of Yahoo, Maktoob Arabic Nasry Esmat, the flag held in the Lindt Cafe could not belong to Jabhat al Nusra as it does not feature the group's name. Likewise, cannot be an ISIS flag as the black flag of Islamic State has the message in black on a white background.
