AGL can probably explain it better but each day, gas or electric you sort of get a first amount, like an allowance, in the case of electricity after you use that up the one is slightly more expensive, called next peak. It may be quoted as per month/3 months or per annum but it's calculated on a daily basis.
In one case in SA it may be say, Summer Peak, first 4000KWh at a certain rate and then it's called 'thereafter' at a slightly higher rate, in al cases it's per annum, so you have to divide by 365 to get a daily rate. AGL moderators here can ask for you private message them to ask for your account number etc. so they can better explain, but you can also go to your AGL on line section, ask for your current tariff and there it's broken down and you can see your own figures.
If you're being billed according to Peak/Off Peak/Peak Next, it means your local distribution company has structured their pricing so that usage is pro-rated based on the billing period and is generally based on the time of year (not the time of day). We publish all of this information on our website as Energy Price Fact Sheets, which I would absolutely recommend using.
Essentially, every contract has different limits for peak usage, once that limit has been reached, the ongoing usage falls under the first peak next, and once your usage crosses that limit, the ongoing usage from there falls under second peak next and so on.
If you do have any further questions in relation to energy pricing and billing for your specific circumstances, I would encourage you to have a chat with us here.