The cuts won't start until July 9, with the Remuneration Authority saying bureaucratic process around pay cuts was complicated and lengthy.

Ardern today told media that despite the delay, she still believed MPs had led by example on pay cuts.

"One of the frustrations for us is, I did test whether I could just not accept my full pay but I was told 'No'. By law I couldn't do that. So we changed the law, now we're waiting for it to be implemented by the Remuneration Authority."

Ardern said they'd canvassed all the options, including giving part of their salaries to charity, but that opened up "wider issues" around who MPs chose to support.

"Frustratingly, the simplest option was to change the law but unfortunately that's what is taking the longest."

Because of the way MPs' pay works, the Remuneration Authority has the final say on pay levels. The bill made changes to the law governing MPs' pay to make the temporary up to 20 per cent six-month pay cut.

Ardern's pay will drop from the equivalent of $471,049 a year to $376,840 for six months, according to the amended rules around MPs' pay.

The Deputy Prime Minister's pay also drops by 20 per cent; Winston Peters goes from the equivalent of $334,734 a year to $267,788.

And all ministers go from the equivalent of $296,007 a year to $236,806 – also a 20 per cent reduction.

MPs who are not ministers, party leaders or chairs of select committees have taken a 10 per cent, $16,000 pay cut – down to the equivalent of $147,565 a year.

Act leader David Seymour, who had been calling for MP salary cuts before Ardern announced them, said a 10 per cent cut for non-ministers was "a pathetic outcome".

"I've said all along that all MPs should take a 20 per cent pay cut for six months. That's what I'll be doing.

"MPs should now take the issue into their own hands. I'll be asking the public which organisation I should donate the remaining part of my salary to."