(a) Prime cost method:
Decline in value = cost * (1 / effective life) = $30,000 * (1 / 5) = $30,000 * 0.20 = $6,000

(b) Diminishing value method:
Decline in value = cost * (2 / effective life) = $30,000 * (2 / 5) = $30,000 * 0.40 = $12,000

Controlling authority: Division 40 of the Income Tax Assessment Act 1997 (ITAA 1997), which sets out the uniform capital allowance (decline in value) rules for depreciating assets.