Welsh Water said levels are “around where we would expect for the time of year”.
The firm said it was pumping an extra one billion litres of water a day into its system to meet hot weather demands.
It said customers could use as much water as they need – but should use water wisely.
The warm weather reached a record high of 32.6C in Porthmadog in Gwynedd at the end of June and the heatwave has continued around the country into July.
While Welsh reservoirs may not be at drought-time levels – they are noticeably lower after June became the warmest month on record.
The Llyn Brianne reservoir is fed by the River Towy, and straddles the borders of three counties, Powys, Carmarthenshire and Ceredigion.
Images clearly show parched areas of the reservoir, which was built to control flow on the Towy, helping provide water to a large part of southern Wales.
Welsh Water, which is responsible for supplying 1.2 million homes and well over 100,000 businesses, would have to enforce measures to deal with drought conditions across most of Wales.
How it responds is set out in its Drought Plan, agreed every five years with the Welsh Government, and governed by strict regulations.
According to the plan, there are three triggers which mean Wales is entering a period of drought: Lack of rain, reservoir levels, and demand for water.
Rainfall: The plan states that if rainfall falls below “significantly dry month” levels for two months in a row, early stages of the drought plan liaison with Welsh Government and Natural Resources Wales (NRW) would begin.
Reservoir Levels: Water levels are constantly monitored and have marked points that would indicate a “developing drought” situation, followed by drought and severe drought indicators.
Elevated demand levels, especially in areas not served by reservoirs, act as drought indicators, and taken into account with weather conditions and forecasts, could trigger drought action.
The drought plan sets out actions for three scenarios:
Developing drought: The message is “use what water you want – but don’t waste it”. Temporary usage bans, such as hose-pipe bans would be prepared. They need to be approved by Natural Resources Wales.
Drought: Temporary usage bans brought in – as the public is urged to conserve supplies.
Severe Drought: Non-essential water use bans introduced. The public asked to “think carefully about the water you use”. As a last resort, Drought Orders restricting supplies could be introduced.
Welsh Water, a not-for-profit firm, has insisted it is a long way from where Wales was back in 1976.
Managing director Peter Perry, said: “The water levels in our reservoirs is around where we would expect for the time of year and we don’t have concerns about the water levels across the area we serve.
“But customers are obviously using much more water in this hot weather and so we need to ensure that we can get the water to customers. Our staff are working day and night to help spot and repair leaks and using tankers to put extra water into the network.
“We always ask customers to work with us. If they spot a leak, please let us know and be mindful of how much water they are using. While encouraging everyone to stay safe and drink water while it’s hot, we should always avoid wasting water so that we can manage the demand for everyone.”