New Green MP Ricardo Menendez March is in managed isolation after ignoring official Government advice against travelling overseas during the Covid-19 pandemic.
The politician, who grew up in Tijuana, Mexico, returned to New Zealand on February 1 and is now nearing the end of his mandatory stay.
The Green Party said the trip was for "serious personal family matters" and he had sought permission before making the trip.
He left New Zealand on December 13 and returned on February 1.
"He followed the appropriate processes to return," a spokesperson said.
However, the trip flies against warnings from the Government's Safe Travel advisory, which says "do not travel" for all overseas destinations.
"We currently advise that all New Zealanders do not travel overseas at this time due to the outbreak of Covid-19, associated health risks and widespread travel restrictions," said the Ministry of Foreign Affairs and Trade website.
According to the World Health Organisation there have been nearly 2 million cases and 166,200 deaths since the outbreak started in Mexico.
In the latter half of January the Latin American nation was experiencing a fresh wave of Covid cases with more than 122,555 people infected and a worrying 20 per cent spike in new cases in the week of January 18.
The Taxpayers' Union is questioning the judgment and secrecy around the trip, taken during the summer break.
Spokesman Jordan Williams said there might be valid reasons behind the politician choosing to travel overseas, such as a family emergency, but it was not acceptable New Zealanders had not been informed.
The idea that a politician could fail to turn up to work because they were secretly in MIQ was incredible.
"If true, it is beyond belief that a statement wasn't put out to at least give the public the heads up that the MP was on personal leave.
"If indeed we are paying a Member of Parliament to sit in an hotel because they chose to take a trip overseas, that would amount to arrogance in the extreme," said Williams.
"Putting aside the needs of other New Zealanders desperate for limited MIQ spots, anyone in the private sector would lose their job for this kind of abandonment of employment."
Williams said for any politician choosing "to skip the country" during a literal pandemic called into question their judgment, fitness for office, and financial credibility in accepting a pay cheque for a job they were clearly unwilling to do.
"Except in the most extraordinary of circumstances, given we pay MPs handsomely, it's not unreasonable to expect MPs to hang around during a global pandemic."
Williams also claimed the politician arrived back in New Zealand at the start of the month after unsuccessfully applying to jump the queue for an emergency MIQ spot in January.
He alleged the application was turned down by Covid Response Minister Chris Hipkins.
But a Greens spokesperson rejects the criticism.
"Ricardo returned to his home of origin for serious personal family matters.
"He followed the appropriate processes to return."
The spokesperson added they were looked forward to seeing him upon his return.
Menendez March has been approached for comment.