Play DS games from any region on the 3DS

The 3DS, unlike the Nintendo DS before it, is region-locked. There are 3 regions: North America, Japan and EU/Australia. So if you buy your 3DS in Australia, you'll need to purchase games from either the EU or Australia as well. Nintendo explained their reasoning in the following statement:

“Nintendo 3DS hardware is available in three versions: Japanese, American and European/Australian,” Nintendo told us in a statement.

“Nintendo has developed different versions of Nintendo 3DS hardware to take into account different languages, age rating requirements and parental control functionality as well as to ensure compliance with local laws in each region. Nintendo 3DS also offers network services specifically tailored for each region.”

It went on: “Additionally we want to ensure the best possible gaming experience for our users and there is the possibility that Nintendo 3DS software sold in one region will not function properly when running on Nintendo 3DS hardware sold in another.”

-Source

Out of the box, no. The Nintendo 64 has a region lockout chip which prevents NTSC (Japanese and US) games from running on a PAL (European) machine.

However, through the use of third-party devices, most games should work. I personally own a N64 Passport Plus which I use to play Hey You, Pikachu! on my own N64. It basically works by using a second, local cartridge to authenticate with the lockout mechanism.

I don't have first hand experience of this, but according to that Wikipedia article, some games won't work even with this - presumably, they perform additional hardware detection, so they might be hard to do anything about. It may be possible to somehow bypass these through Action Replay codes; I don't know exactly how these games perform those checks.

You could also buy a Japanese N64 and bring that home. You'll need a power converter to make it run on 230V, and a TV which will accept the input, but you could run any Japanese game that way, and any US game if you remove the plastic tabs.

As an alternative, if you have a European Wii, at least some of the games might be available on the Virtual Console, saving you the trouble of messing with adapters.