The Interislander would also be putting on additional sailings to provide extra capacity for the extended period of homeward travel.
Interislander executive general manager Walter Rushbrook said only people travelling home should be making bookings, either by phone or through their website.
"After the period for travel home ends at 11.59pm tomorrow, and if we are still at Level 4, members of the public will not be allowed to travel on the ferries unless they are essential workers or have a valid reason for travel, as permitted by the Ministry of Health."
On Tuesday the Government put the entire country into alert level 4 after the discovery of a community case of Covid-19 earlier this week.
Kiwis who were not at their usual place of residence were initially given 48 hours to get home, a window that was set to close at midnight tonight.
Earlier today, NZ Herald Wellington business editor Hamish Rutherford said he had been trying to get home to Wellington from Queenstown, but there had been no flights available within the Government's grace period.
Their original flight had been booked for Sunday.
"We're booked on the first flight we could get a booking on … from Queenstown to Wellington on Friday but we just don't know if that plane will be allowed to take off.
"We're just waiting to see if Air New Zealand can get an extension to fly out of Queenstown for another day or two."
If they were still unclear on whether they could depart Queenstown on Friday, Rutherford said they might have had to drive the five hours to Christchurch in the hope of getting on a flight there today.
Air New Zealand has reportedly organised more than 800 flights for 50,000 passengers throughout the 48-hour window.
But it said when the grace period ends it will adjust its domestic schedule in line with alert level 4 restrictions across the country.
The airline would operate a limited domestic scheduled to allow for essential travel only, from Friday, August 20 to Tuesday, August 24.
It said customers booked to travel on Friday would receive a cancellation email, and their bookings would be held in credit.
"During alert level 4, travel is restricted to essential services only," an update from the airline said.
"Customers who are travelling should check they are eligible to travel under alert level 4 on the Government's Covid-19 website.
"These customers will also need to carry a letter to prove they are travelling for essential services."
It included two daily return services between Christchurch and Wellington, and one between Wellington and Nelson.
Until Saturday there would be one daily return service between Wellington and Auckland, and from Saturday this would drop down to just four times a week.
