Average processing times are three months worse than October last year.
Nine in 10 applicants receive a decision on their citizenship within 16 months, up from 14 months in October.
The worsening waits follow the axeing of hundreds of processing staff, a boom in lodgements and a shift to automation.
Greens senator Nick McKim said the ballooning delays reflected a lack of respect for aspiring citizens, laying blame for the blowout at management for shedding staff.
Senator McKim said the delays were impacting on crucial life decisions around employment, housing and education.
"People want to be able to make plans for their lives and for many people that's not currently possible because of the delays," he told AAP.
"People who want to make such an important commitment to Australia by becoming citizens should be respected by having their applications dealt with in a reasonable timeframe."
The blowouts come as the Turnbull government pursues tougher hurdles to citizenship, including extending the waiting period for permanent residents to become citizens to four years.
A Home Affairs spokesman said there was a growing number of people seeking citizenship each year, and the department had a duty to thoroughly assess each application.
"Resolving these complex issues is often neither straightforward nor quick, as the decision-maker is faced with a set of circumstances unique to each case," the spokesman said.
Home Affairs has been allocated more than $100 million to overhaul its visa and citizenship processes, with most categories experiencing double-digit growth.
Appearing before a Senate hearing last year, Home Affairs secretary Michael Pezzullo conceded wait times were likely to worsen as his department pursued large-scale automation.
Justifying a decision to axe 355 officers, Mr Pezzullo said shifting to automation - not hiring more staff to process paper files - was the only way to manage the workload.
But Brooke Muscat-Bentley, from the public sector union, said it was bizarre for the federal government to continue cutting processing officers despite the steady rise in applications.
Staff were dealing with frustrated and anxious customers and an increasingly difficult workload.
"Putting the system under so much unnecessary pressure benefits no-one," she said.
The union is concerned the government is "deliberately running down" citizenship processing as a pretext to privatise the operation, as it is doing with visa processing.