You can not extend 457 or 482 visa, but in some situations you can apply again for next visa.
If you are granted a 457 visa after 18 April 2017 off the STSOL list, it can only be granted for 2 years. There will be one (only) further extension allowed after that for another 2 years. ... You will be able to be apply for a permanent 186/187 visa after 3 years on a 457 visa if that company decides to nominate you.
If your occupation make you eligible under the short-term stream then maximum time period that your TSS visa will be approved for is two (2) years and you can not apply again.. If your occupation makes you eligible under the medium-term stream, then the maximum time period that your TSS visa will be approved for is four (4) years and you can apply again for 2nd 482 visa.