The address was scheduled for 8 February.
However, Mbete says it had become clear that it was unlikely that a joint sitting of the two houses would be uneventful if it was held this Thursday as planned.
She adds that Parliament took into account views of political stakeholders, society and concerns in the political environment.
Mbete says that as Parliament was preparing to write to President Jacob Zuma to request the postponement of his address, he was already doing the same.
“When we met the president this afternoon we then learnt that he was already writing to Parliament to ask for the postponement.”
The Presidency has released a statement, confirming that Zuma wrote to the Speaker to request the postponement.
Spokesperson Bongani Ngqulunga said: "The joint sitting was scheduled to take place on Thursday 8 February 2018. The president has requested the postponement due to certain developments which make it not conducive to successfully hold the sitting and deliver the State of the Nation Address. The Presiding Officers have acceded to the President’s request."
Standing on the steps of Parliament, Mbete says a new date will be set as soon as possible, adding Parliament is working to ensure the budget speech goes ahead on 21 February.
Mbete says they have also been in contact with Deputy President Cyril Ramaphosa's office.
“We are in continuous interaction with the president of the ANC because he is also the leader of government business. So, of necessity, when we are dealing with issues dealing with Parliament, we must be talking to his office."
She says it's hoped that the address will not be postponed for more than a week, so as not to affect the tabling of the Budget.
She says the decision has been taken in the best interest of the country.
“We have since looked realistically at the developments and we thought that we need to create a room for establishing a much more conducive political atmosphere in Parliament.”
National Council of Province’s Chair Thandi Modise says a new date will be announced soon.
“We have indeed considered [the postponement]. We do have a proposal not to postpone by more than a week. So, if we can, a week will be sufficient.”
Modise says the decision was made after listening to all points of view.
“We need also to consider the mood of the country and we need to then have this consultation with the president of the republic. So, that we are satisfied in our minds that there is no prospect of a very productive Sona.”
The ANC's Jackson Mthembu has given his immediate reaction, saying the party welcomes the postponement.
The rand has firmed slightly off the back of Tuesday afternoon's announcement.
This article first appeared on EWN : Mbete: Sona 2018 postponed, no new date set