BISMARCK, N.D. (AP) — A state board agreed Thursday to hire a consultant for advice on how to invest a rapidly growing, $500 million North Dakota oil tax fund that was established by voters less than two years ago.

The Legacy Fund gets 30 percent of North Dakota's oil tax collections, which have been rising along with the state's production. The state averaged 660,332 barrels daily in June.

Since oil revenues began pouring into the fund last September, the money has been parked in bonds, its least risky investment option but also the choice that provides the stingiest earnings. The fund, which now totals $489.8 million, has had an investment return of just over 1 percent.

The North Dakota Legislature created an advisory board to explore investment options for the Legacy Fund and a separate "rainy day" fund, called the Budget Stabilization Fund, that holds almost $400 million.

Members of the board, headed by state Sen. Randy Christmann, R-Hazen, voted unanimously Thursday to support hiring a consultant to advise them about how much of the money should be kept in bonds, and how much should be put into stocks and other, riskier investments that could potentially earn greater returns.

John Lewis, a portfolio manager for Dorsey Wright & Associates, a money management firm based in Pasadena, Calif., said keeping the fund exclusively in bonds carries its own drawbacks.

"There's a lot of risk in holding all bonds, or being too conservative, because inflation will eat away at those returns," Lewis said.

The board's members include Christmann and three other legislators; Tax Commissioner Cory Fong; Eric Hardmeyer, president of the Bank of North Dakota; and Pam Sharp, the state budget director.

Darren Schulz, the interim chief investment officer of North Dakota's Retirement and Investment Office, recommended the consultant option. He estimated the work would cost about $60,000 and take about two months to finish.

Schulz will contact prospective consultants and present recommendations to the advisory board. North Dakota's Investment Board, which supervises the Retirement and Investment Office and oversees state and local government employee pension funds, will do the actual hiring.

Christmann and other board members said one unpredictable future element is whether the Legislature will be willing to spend money from the fund, rather than let its value continue to build.

The voter-approved constitutional amendment that established the fund in November 2010 says the Legislature may not spend any of its revenues until June 2017. Once lawmakers can dip into the fund, the amendment says they may not tap more than 15 percent of its principal over two years.

Christmann said if the money were available now for the Legislature to spend, he is confident it would stay put, because of North Dakota's strong economy and state budget surpluses. However, the economy could go sour and the makeup of the Legislature could change dramatically, he said.

"Even though we don't intend to spend it, we have a responsibility to make sure that there is a portion of this that's available ... without taking a great big loss in the value of the investment," Christmann said.