They say the booking system for places in border facilities has not had this level of vacancies since October last year.
Except for travellers cancelling their reservations, no vacancies were expected until June.
"For the first time in a number of months, demand for managed isolation vouchers is down and a number of vouchers are available in April and beyond," a managed isolation and quarantine (MIQ) spokesperson told RNZ.
"MIQ wants to make the most of the rooms in managed isolation and get as many New Zealanders home as possible. For anyone looking to come home, we encourage them to get on to the Managed Isolation Allocation System (MIAS) website and book their space now.
"We have not had this level of MIAS vouchers available in the upcoming few weeks since late October 2020, when we saw a significant increase in demand ahead of the summer months."
Capacity in the next fortnight is forecast to fall as low as 3500 rooms - about 1000 lower than the system's operational capacity. Half of the rooms at the Pullman Hotel - 150 - are still out of action after guests contracted Covid-19 there in January.
Empty rooms come at a cost to taxpayers, who paid $499 million towards managed isolation last year. MIQ did not explain the possible reasons for so many vacancies emerging in April.
Seasonal demand from New Zealanders, who are less likely to visit in autumn and winter, could be one explanation - although managed isolation had been booked out until 31 May. When MIQ released June and July spaces earlier this month the website crashed because there were about 1m hits. But following that rush, vacancies have remained.
Managed isolation has been mandatory since November, causing chaos when it started - as not everyone who had bought flight tickets knew about it - and heartache when the places started running out in the run-up to Christmas.
The managed isolation booking system was getting about 100 formal complaints each week and a wait-list was under consideration to manage demand.
The opposite problem occurred in October, when the average daily vacancy was 1291.
Overseas workers
New government figures are forecast to show that at least one in 10 managed isolation rooms has been allocated to overseas critical workers since January.
It came at a time when New Zealanders wanting to return had faced huge challenges getting accommodation at the border.
A government spokesperson said it had introduced a target to allocate 10 percent of spaces in managed isolation facilities to honour an election pledge.
"The target was put in place from 1 January and is being monitored on a quarterly basis. Information for the first quarter of 2021 will be available shortly but current indications are that, on average, at least 10 percent of MIQ capacity is being allocated to critical workers - in line with the government's election manifesto commitment."
That election pledge was among a slew of announcements in one week in September.
Temporary visa holders, such as workers and students, are being charged more towards their stay from last week to try to recoup a greater share of the costs.
