Broadband users in more than 400 suburbs across Australia risk being disconnected from the internet early next year as their deadline to switch to the National Broadband Network may be closer than it appears.

Households that have delayed switching to the $57 billion network may have as little as five weeks left to make the move as some internet service providers plan to pause NBN installations from mid-December.

The Christmas shutdowns, in addition to public holiday delays and rising demand, could see households due to be disconnected in mid-January unable to switch to the NBN in time, and experts warn it could cut off some critical services unexpectedly.

Households are typically given 18 months to move from their existing broadband service to the NBN, with cut-off dates published on the NBN Co website.

But many people are delaying their transition until the last moment.

NBN Co's latest figures show more than four million households and businesses are yet to use to the network even though their suburbs are deemed as "ready to connect".

And News Corp has seen letters sent to ADSL users, warning that from "December 14 through mid-January … no activity will be undertaken on NBN services needing cut over to the National Broadband Network".

But NBN Co state media relations executive manager Jane McNamara said the company's installers would still be available over the Christmas period, though individual internet providers may choose to limit or suspend new connections.

"Our installation and connection teams will be on hand over December and January to complete orders and we encourage people to place their orders as soon as possible to avoid any holiday rush," she said.

"It's important to remember that making the switch is not automatic."

The average time to connect a broadband user to the NBN was currently 10 days, she said, and appointments were not available on public holidays, which could delay December connections.

Independent telecommunications analyst Paul Budde said cutting off broadband users who could not make the transition between late December and early January "would be an unfair situation".

But Mr Budde warned few households would be granted "leeway" now the project had passed its completion date, and broadband users should take careful note of their disconnection deadline.

"In the past, NBN Co has been reluctant (to disconnect areas) but from now on they're going to do that," Mr Budde said.

"They need the money from customers because their financial situation is not that fantastic.

"If your suburb has reached that position then that's it - you take it or leave it."

Households in areas where ADSL and HFC internet connections were being cut could also lose more than just internet access, Calamity cyber security expert Daniel Lewkovitz warned.

Monitored fire alarms and security systems, lift emergency telephones, and some medical alarms and emergency call buttons could also be disconnected in the transition.

"Sometimes people find out the easy way - they might get a phone call from a monitoring company such as ours saying their monitoring system hasn't checked in," he said.

"But their system will just stop working, and if it's a life safety system or a security system, that's a problem."

Mr Lewkovitz said many households ignored early warnings about NBN disconnection dates and dismissed unaddressed NBN notices as junk mail, but they should put in effort to "get on top of this now" or risk serious consequences.

Mr Budde said internet users who found themselves unable to switch to the NBN in time could opt for mobile broadband options, including 5G services offered by Optus and Telstra, but warned they would not suit every user in the long-term.

"If you are using Netflix every night and you have a couple of kids then it might not be an option but if you are not a big user, 5G is definitely worthwhile investigating," he said.

The National Broadband Network was due to be completed in June this year. There are currently 7.7 million households and businesses actively using an NBN service, and more than 21,000 premises yet to be connected.

Originally published as Broadband cut-off: Queensland homes to be disconnected