Bonsai do need to be kept in strong vigour and health. There are numerous fertilisers on the market that will achieve this for your Bonsai. You can use slow release granular fertiliser as one way of feeding your Bonsai.
Usually two applications during the year will for most varieties of plants will be sufficient. They can have one application when they are re potted and then a follow up application sprinkled on the surface 4 to 5 months later.
If you have Australian Native plants be sure to use a fertiliser low in Phosphorus or use a slow release fertiliser labelled for Australian Natives.
Another way of fertilising is by using organic liquid fertilisers. These can be used at the recommended strength once a month.
If you are applying the liquid fertilisers via a watering can it is always advised to water your plants well prior to applying the liquid fertiliser.
Alternately you could mix some liquid fertiliser in a large container of water and submerge your Bonsai up past the soil level and when the bubbles stop the soil has reached saturation and you can pull the tree out of the container.