Four people have already been turned away from their flights to New Zealand for not having a managed isolation voucher.

The online Managed Isolation Allocation System (MIAS) was launched in early October and the need for a voucher to enter New Zealand managed isolation facilities came into force yesterday.

A Managed Isolation and Quarantine spokeswoman said so far four people had already been stopped from boarding their flights to New Zealand because they did not have a voucher.

The voucher system was brought in to manage capacity during the lead-up to Christmas because the Government said there were not enough health workers and police to cope with the number of travellers arriving into New Zealand who must complete 14 days of managed isolation to stop the spread of Covid-19.

The system was now booked out until December 20, locking hundreds if not thousands of New Zealanders out of a Kiwi Christmas with loved ones.

It was not clear where the four travellers turned away were flying from or whether they were individual travellers or a family, and whether they were now stuck in transit or had secured new flights for a later date.

The spokeswoman said the allocation system had been operating since October 5 and a grace period of one month was provided for passengers to use the voucher process.

She said a campaign advising travellers of the system has been running since September 25.

"MIAS helps us manage the timing of people entering New Zealand so we can guarantee their place in a managed isolation facility, which is necessary to keep them and all New Zealanders safe.

"At this time there are currently no vouchers available until December 20. The MIAS is now fully booked during this period, which means there is no capacity for additional bookings in managed isolation facilities."

The spokeswoman said the four travellers turned away would now need to reschedule their flight and secure a place in managed isolation where space was available.

"The reality is that there is finite capacity within the MIQ system. New Zealanders can still come home but possibly not on the dates they would prefer."

On the Facebook page for the Ministry of Business, Innovation and Employment, which runs the allocation system, travellers complained of not being able to match flights with voucher dates.

One man, who had flights from Malaysia, said he secured an allocation for December 7 and then booked his flight, as was the recommended process.

"Returning to enter my flight details I found that my Malaysia Airlines flight was unlisted [no, it's not because of a codeshare number].

"I rang your call centre and they said to wait a few days, and not to worry about if my allocation lapses."

After three days the man said his allocation date and others around it were fully booked leaving him with a confirmed flight and no voucher.

"Malaysia Airlines is only operating intermittently and if I don't take that flight, it will be a three-month wait till their next flight in early March."