There are two main areas of Government assistance when buying solar panels.
The first area is similar to a cash rebate and it falls under the Federal Renewable Energy Target (RET) legislation.
The RET requires wholesalers of electricity to purchase a percentage of the energy from renewable energy sources.
The mechanism used to measure the percentage of renewable energy an energy wholesaler purchases is structured around them surrendering certificates which can by purchased from solar energy system owners.
The RET is discussed in more detail here.
The second area are state based feed in tariffs. A feed in tariff is the rate at which the energy retailer pays for the solar electricity they purchase from your solar system. Sometimes these tariffs are priced at a premium and at other times there is no regulation requiring energy retailers and distributors to pay for the solar they gain from your solar system.
Feed in tariffs are discussed in more detail here.