"You're putting a lot of faith in where the cows come from, and where they've been. There's absolutely nothing you can do, just make sure your own paperwork's up to date," he said.
The only other option was to closing herds and not move cattle.
"My big hope is that we can just close the gate at the [Auckland] harbour bridge," Mr Cullen said, adding that there could be an opportunity for Northland if the region remained free of the disease.
Mycoplasma bovis was confirmed in the Waikato, the furthest north it has been found, last week, taking the number of infected properties throughout the country to 39. More than 22,000 cattle have been culled, or soon will be, on MPI orders.
The disease was first detected in South Canterbury in July last year. It is not known how it arrived in New Zealand.
Agriculture and Biosecurity Minister Damien O'Connor said last week that a decision on the next steps to deal with the disease would be made soon.
"We are all committed to making a decision about the next steps within the next couple of weeks," he said.
"We talked about phased eradication and long-term management. It is a difficult choice that we will make together once we receive more advice from the Technical Advisory Group in the coming days.
"Over the next few weeks farmers who are not under controls are allowed to move stock, but they must adhere to their legal National Animal Identification and Tracing requirements, and record animal movements. If you are concerned about moving your stock then be prudent, seek advice from your industry groups and MPI. The same goes for sourcing feed."
M. bovis is spread between cattle by physical contact, but is not a danger to humans. Milk and meat from infected animals can be consumed without risk. It cannot be transmitted to sheep, but goats are thought to be able to carry and transmit the it.
THE FIRST PRECAUTION
A Ministry for Primary Industries' Notice of Direction (NoD) is issued when an inspector or other authorised person believes that movement of stock and other risk goods from a property might pose a risk of spreading Mycoplasma bovis.
It does not necessarily mean that animals on that property have been tested, or if they have, that test results have been received.
The NoD doesn't restrict movement of stock or goods on to the farm, but means they can only be moved off the farm with a permit. Other precautions, such as cleaning and disinfecting vehicles, may also be required.
