Either Melanotaenia duboulayi or Melanotaenia fluviatilis.
If you bought them in Australia as M. duboulayi, then that is what they will be. They will need time to colour up and show their full potential.
They do best in groups of at least 6, and you can mix different species together and they will all hang out. Have even numbers of males and make sure they are the same size otherwise the big males will bully the small ones.
The male has the longer pointy dorsal and anal fins, the female has shorter more rounded dorsal and anal fins.
Make sure they get lots of plant matter in their diet.
If they get sick, do a huge water change, post a picture on here, and add salt (2 heaped tablespoons per 20 litres).
There's more info about rainbowfish at the following link.
A website dedicated to Rainbowfishes of Australia and New Guinea
rainbowfish.angfaqld.org.au
If your interested in rainbowfish, you can look up ANGFA (Australian New Guinea Fishes Association). They are online and have groups in most states around Australia, with monthly meetings and auctions.