By Alice Uribe

SYDNEY--Genworth Mortgage Insurance Australia Ltd. signaled the loss of a major contract after National Australia Bank Ltd. sent notice that it won't renew its supply and service deal.

The lenders mortgage insurance company said Wednesday that the contract with NAB was worth 12% of gross written premium in the 2019 fiscal year. It will expire November 20, Genworth Australia said.

"We are disappointed in the outcome given our longstanding relationship with NAB and would have welcomed the opportunity to continue the partnership," said Genworth Australia Chief Executive Pauline Blight-Johnston. "We believe we presented a strong offer that reflected the risk of the portfolio while maintaining adequate returns on equity for our shareholders."

According to Genworth Australia, NAB still has the option to extend the supply and service contract for up to four periods of three months each.

Genworth Australia recently renewed a contract with its largest customer, Commonwealth Bank of Australia, for three more years. The new contract is effective January 2020.

Write to Alice Uribe at alice.uribe@wsj.com