Australian Agricultural Company chief executive Hugh Killen has resigned after more than four years in the top job.

His resignation is effective immediately.

A statement from the company to the Australian Stock Exchange said it was the right time to begin an orderly executive leadership transition.

"The board and I would like to take the opportunity to thank Hugh for his significant contribution to the business and recognise the progress that has been achieved under his leadership," the statement reads.

"Hugh has steered AACo through some of the worst droughts and floods Australia has ever seen, the global impacts of COVID-19 and market fluctuations which occurred as a result of market impacts."

AACo posted an operating profit of $49.9 million last month, with the value of its herd increasing to nearly $200 million. 

The company's herd has increased to about 382,000 head and its property portfolio was valued at $254 million in May. 

Chief operating officer Dave Harris will take up the role of acting chief executive. 

"The board will now conduct and executive search process that will include internal and external candidates in the domestic and international market," the statement reads. 

Andrew Forrest increases investment 

It comes after mining magnate Andrew Forrest's investment company Tattarang last week made a significant increase in its shareholding in AACo.

Tattarang started buying into AACo last year, and late last week purchased another $122 million in shares, bringing its holdings to 17 per cent.

The investment is just below the 20 per cent threshold whereby a takeover bid must be made.

The latest share purchase likely came from Heytesbury Cattle Company's 7.5 per cent stake in AACo, according to the Australian Financial Review.

Tattarang declined to comment on the AACo investment.