Complete isolation may not be feasible. You can communicate using something as simple as a beam of light, and you can't completely block those without blocking the sun as well. Your story probably doesn't need true isolation, though. You merely want your colonies to operate without influence from Earth, and there are several ways you can do that.
Unpredictable solar activity
The sun has entered a period of semi-instability where it is prone to severe and unpredictable sunspot activity and coronal mass ejections. This causes waves of highly-charged particles to flood the inner solar system, scrambling any communication signals and potentially disabling the electronics of any spacecraft in transit. Anything planet-side is protected by the magnetosphere, but the CMEs happen so frequently that there's no way to get a spacecraft between planets before the next one hits. Communication satellites don't last very long in orbit, and the streams of highly-charged particles create so much such electromagnetic noise that ground-based interplanetary communication systems aren't effective. Your colonies were established before this instability started, and now they're effectively cut off.
Earth doesn't interfere or communicate with your colonies because they don't want to. Perhaps the colonists were a group of people that Earth wanted to get rid of. The people of Earth rounded them up, shipped them off to the New Australia colony, and tried to forget they ever existed.
A variation of this is that there was a major incident of some sort as the colonists were on their way to their new planet. Their ship lost communication, and authorities on Earth believe that the ship was destroyed. Your colonists survived, however, but Earth doesn't know that so they never make any attempt to contact them. The apparent "tragedy" elicits such a strong emotional response from around the world that Earth's space programs never attempt to send another colony ship. The colonists are perfectly alive but on their own.