Margins are essentially collateral - in the form of cash or share holdings - that you need to provide to ensure that you can meet your obligation in the event of an adverse market movement. Margins are only payable by Option writers (sellers); Options buyers do not need to pay margins.
Margins are calculated daily by ASX Clear, the clearing house for all shares, structured products, warrants and ASX equity derivatives.
Profitably Trading Options in Australia - Understanding Volatility
It calculates margins on all the written positions in your ASX Options portfolio, and the amount payable is the net amount on all positions.
The total margin required for open positions in a CommSec Options account are as follows:
Total Margin = (1 x Premium Margin) + (2 x Risk Margin)
To estimate your total required margin you can use the ASX Margin Estimator or refer to the ASX resource, Understanding Margins.
It is important that you monitor your margin requirements, as they vary daily and must be covered at all times.
Please be aware that CommSec conducts daily stress tests on short positions which may result in your collateral requirements being higher than your daily total margins.