So it's definitely worth getting it out of the drawer and putting it to better use, finder.com.au's money expert Bessie Hassan said.
"Travellers should dig out their spare holiday cash because it could be worth more than they realise," she said.
"It's very common for people to think it's too much hassle to exchange leftover foreign currency and don't bother converting it back to dollars.
"I would advise anyone with a stockpile of foreign notes and coins lying around at home to explore what it might be worth."
OK, so what do you do about it?
The first thing to remember is currency exchange services don't generally accept foreign coins (and notes have to be in good condition) — so you have to get rid of those coins.
Do you know there are donation boxes at airports all over the world that collect your spare foreign coins and donate them to worthy charities? You probably don't — as less than 5 per cent of travellers actually use them.
Airlines also accept leftover change on board and donate them to good causes.
"If you're already back home and want to use leftover foreign cash to support a charity, you can donate it to charity like UNICEF," Ms Hassan said.
Many international charities like UNICEF take donations in any currency and have transfer arrangements with highstreet banks, so that they receive 100 per cent of any donation.
Notes are worth converting back to dollars, and many currency exchange services — as well as some banks — will buy back your holiday money.
But don't trade them in at the first currency exchange you see.
"Comparing a range of specialist currency exchange services online is probably your best bet," Ms Hassan said.
"Look at the 'buy' exchange rates as this will help you lock in the most competitive rate when converting the currency back into dollars. The service may also be offered through your bank, but make sure you check the costs associated with doing this as will vary depending on the provider."
Ms Hassan said hotels and airports were the worst place to exchange your currency.
"While it's more likely that you'd exchange foreign currency at a hotel or airport in a foreign destination, it's still important to remember to avoid this once you've touched down on home soil," she said.
"Exchanging foreign currency at hotels or airports will likely mean an unfavourable exchange rate."
Another great idea is to keep an eye out for anyone you know who could use your foreign money.
Got a mate who's going to Bali, and you've got a stash of Indonesian rupiah? Trade 'em.
"Using the wholesale exchange rate, available online, means that both of you will usually get a better deal than if you went to a currency exchange service," Ms Hassan said.
Increasingly, though, our leftover holiday money is stored electronically on travel money cards. Those travellers come home with even more unspent foreign currency (NZ$285 each, on average) than those relying on cash.
Of course they can easily transfer this money back into New Zealand dollars, but the exchange rates are likely to be higher.
Finder.com.au advises funds transferred to your card will be subject to the currency conversion fee on the day, which are often similar to credit card conversion rates.
"As a general tip, don't transfer more cash into a foreign currency on your card than you expect to use," Ms Hassan said.
"Often, you'll lose money by converting to an international currency and converting back again."
