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WELLINGTON, Aug 29 (Reuters) - A serious bacterial cattle disease has been identified at three more farms in New Zealand, raising the number of affected farms to six, the country's Ministry for Primary Industries (MPI) said on Tuesday.

Two of the farms that reported positive results for the Mycoplasma bovis disease belong to the Van Leeuwen Dairy Group, which also owns the first two farms to report the bacteria in July.

The owner of the third unnamed farm in the country's South Island had received a small number of calves from another infected farm, the ministry said in an emailed statement.

"We fully expect to find more infected properties as we continue our tracing and testing programme," MPI Response Coordinator David Yard said, adding that there was no evidence the disease was spreading without close contact to infected animals.

Mycoplasma bovis is common in many countries and can lead to conditions such as udder infection, pneumonia and arthritis in affected cattle, but does not pose a food safety risk or any risk to humans.

News of the outbreak in July briefly knocked the New Zealand currency given the importance of the cattle industry to the economy, with dairy the country's largest export good.

The New Zealand dollar extended losses on Tuesday, falling from $0.7237 to $0.7224.