An Australian family has left New Zealand short-changed despite clutching a handful of coins.

Retiree Mike Eisenblatter and his family from Adelaide have $2.40 in old New Zealand coins, which they say were given as change from places including Te Papa.

When they tried to spend the change they were rebuffed because the old larger coins, phased out in 2006, are no longer legal tender.

Mr Eisenblatter, 59, said he felt duped after discovering he had been given useless coins. Tourists had no way of knowing the coins were not legal tender, he said.

"None of us paid good, hard-earned money to come here and be duped due to our ignorance."

While the dollar amount was small, it was concerning they were given the coins several times. One trader who rejected the coins suggested he had been an "easy target", he said.

After they became aware of the difference in coins, his son Shannon, 15, was also handed the old currency at another shop, but refused to accept them.

"The guy was trying to tell me they were real and I could still spend them," Shannon said.

Mr Eisenblatter said it was up to the Reserve Bank to get the old coins out of the system, and he suggested alerting tourists to the difference. "It may seem like small change, but in the larger scheme of things, if every other tourist assembles $1, $2 or even $5 of this stuff, you can do the math on what might still be out there."

The family returned to Australia yesterday, and had thoroughly enjoyed the visit despite the coins, he said.

A Reserve Bank spokeswoman said old coins could be exchanged for new ones at the Reserve Bank.

Retailers could choose to accept old coins, but in no circumstances should they be giving them as change.

Te Papa is investigating.