A disease which has already led to the culling of thousands of cows has spread "beyond all expectation" in the last six days, biosecurity officials say.

The number of farms under regulatory control for Mycoplasma bovis disease has jumped from 130 properties to 290 over that period, Biosecurity New Zealand head Roger Smith told MPs at a briefing at Parliament this morning.

That meant movement of cattle was being restricted while testing for the disease was being carried out.

So far, 38 properties have tested positive for the disease in the North and South Islands. More than 20,000 cows have been culled.

The first case was detected 300 days ago in Oamaru. But Smith said the number of farms requiring testing had hugely increased in the last week.

It was unclear whether it was a "one-week" blip or whether the disease was reaching epidemic proportions, he said.

Once that was known, officials would be able to advise whether the disease could be eradicated or whether the Government would have to instead focus on containment.

It was not still known where the disease originated from.

Primary Industries Minister Damien O'Connor said it was always expected that more infected farms would be found.

"[However], officials tell me that the numbers will likely exceed their earlier modelling.

"That modelling work is continuing and we will have a clearer picture in the next couple of weeks."