Generally, maximum temperatures occur during the early to mid afternoon. In summer, they can occur as late as 5pm, but in winter they tend to be closer to around 1pm. However, if a cold front moves through or an early sea breeze develops, then maximum temperatures can occur much earlier in the day than this.
Minimum temperatures generally occur around or shortly after sunrise. This time is far less variable than maximum temperatures and will almost always be the case. The exception is if winds pick up substantially during the night, which often tends to break nocturnal inversions and allow temperatures to rise.
When you see a forecast maximum and minimum temperature, this is referring to the highest and lowest that the temperature is expected to get to during that day. It does not say anything about when that temperature is expected to occur. However, this can normally be inferred from the rest of the forecast and the above information about normal times.
17:01 EST Sydney's offshore Sea Surface Temperatures (SST's) have remained above 21 degrees for the last four days.