The Australian Taxation Office's online services will enter a fifth day of being unavailable as technicians from the agency and its storage provider HPE struggle to remedy a faulty storage network.
The ATO's online services - including its main website and customer portals - fell over on Thursday morning.
The cause of the failure was related to the collapse of an HPE SAN (storage area network) in early December last year, which is currently being replaced.
The ATO's technicians - alongside others dispatched from its storage vendor HPE - have been working through each night since Thursday on the "highly complex" restoration, but as yet have been unable to get the systems back up and running.
The tax agency on Saturday advised that "unforeseen complexity" in the restoration process has meant its online services won't be available before Monday 6th February.
"While we are working as quickly as we can, we cannot make services available to the community until the integrity of the system is confirmed," it said.
It is currently replacing the faulty HPE hardware with a new SAN, which it said had arrived on ATO premises and was currently being configured.
"The nature and scope of the ATO’s SAN means that this process of replacing the affected hardware will take some time," it said.
"We fully acknowledge the significant impact this is having on our key stakeholders, who rely heavily on the availability of our systems and services. We apologise for the inconvenience and disruption this has caused.
"While everything is being done to restore our systems as quickly as possible, we cannot make services available to the community until the integrity of the system is confirmed."
HPE has flown in staff from overseas to help fix the problem.
The collapse of the storage network on December 12 - which downed the ATO's entire online presence and several internal systems for two days, and was followed by a second outage the following week - is being investigated by PwC as part of an independent review into what went wrong.
The review will be finalised in March, and runs alongside the ATO's own internal investigation into the matter.
Update 6/2/17: The ATO has advised that most of its core systems are back up and running.
Its tax agent, business and BAS agent portals, as well as its SBR and online services, are now available. Superannuation systems will come online this morning, while the Australian Business Register will be available "shortly".
"Our clients may experience some slowness as further work is undertaken to improve the overall performance of our systems," the agency advised.
"Our focus will now turn to building system resilience to best ensure the stability of our services to the community."