A former clerk in the finance office of the Australian Embassy here pleaded guilty yesterday in federal court to diverting $671,802 in embassy funds into a dummy bank account and then fleeing to Rio de Janeiro and Greece.

John Ian Cowling, who pleaded guilty to interstate transportation of money taken by fraud, told U.S. District Court Judge Barrington Parker that he first devised the scheme to divert the money because "I was trying to prove that I was as good as anyone else and should be in a better position" at the embassy.

The British-born Cowling, who is wanted in England on an unrelated charge of obtaining money by fraud, worked as a $12,000-a-year clerk in the accounting section, reviewing payment claims and vouchers submitted for payment.

Cowling told Parker yesterday that shortly after being assigned to the finance office in July 1981, he warned supervisors that the embassy's system was flawed and that "someone could steal a million dollars from it."

Cowling said he opened an account with a branch of Riggs National Bank here in the name of E Systems Inc., the same name as a Florida defense contractor that sells communications equipment to the Australian military.

Cowling, who faces up to 10 years in prison, said he forged the names of two embassy officials on a voucher last December and had a $671,802 check for E Systems sent to his account here instead of to the company in Florida. The scheme was discovered after the company inquired about the money owed it by the embassy.

By then, Cowling had withdrawn $61,000 in cash and the remainder in a $610,000 cashier's check and flown off with a friend to Rio. He withdrew $183,000 in traveler's checks from an account he opened in Brazil before U.S. officials notified the bank in Rio to stop payment.

The remaining $427,000 was forwarded back to Riggs, but a Brazilian currency devaluation at the time cost the Australians about $100,000. Another $40,000 in cash and jewels was recovered by law enforcement officials from Cowling. In all, the embassy lost more than $300,000.

An embassy spokesman declined yesterday to comment on the plea.

After Cowling learned of the stop-payment order in Brazil, he then went on to Rhodes, Greece, where he opened a discotheque. Greek authorities, acting on an FBI request, arrested Cowling in January and held him for extradition. He was returned here in August and has been held in jail since then.

Parker told Cowling that it seemed he took the money "not only to demonstrate that it could be done but to see that it was done." He said he would sentence Cowling on Dec. 17 and warned him to "expect some more time in addition to what you have served."