A network node (200), a wireless device (202) and methods therein, for communicating or obtaining system information. The network node (200) transmits (2:1) a System Information Block Type1, SIB1, message that includes at least one other SIB than SIB1 as an extension. The wireless device (202) reads (2:2) the at least one other SIB in the SIB1 if the wireless device (202) is capable of reading the extension in the SIB1 message. The network node (200) may further transmit (2:1) the at least one other SIB in a subsequent separate System Information message. The wireless device (202) is then able to read (2:4) the at least one other SIB in the subsequently received System Information message if the wireless device (202) is not capable of reading the extension in the SIB1 message. Thereby, if the wireless device is capable of reading the extension in the SIB1 message it will acquire the system information earlier than if the other SIBs are only transmitted in less frequent System Information messages. If the wireless device is not capable of reading the extension in the SIB1 message, it can still acquire the other SIB(s) from the subsequent System Information message.