AGL made an offer to buy Vocus in May but backed out on the last day of the month when the terms of due diligence could not be agreed.
Last week, Swedish equity firm EQT Infrastructure pulled out of a bid for Vocus that valued the company at $3.3 billion or $5.25 a share.
In an announcement to the ASX on Tuesday, Vocus chief executive Kevin Russell said: "After last week's announcement that discussions with EQT Infrastructure had ceased, AGL returned with a non-binding and indicative proposal to acquire Vocus for $4.85 per share.
"However, we have been clear that the board will always act in the best interests of our shareholders to engage with credible parties that bring forward proposals that are worthy of further consideration.
Vocus said the indicative proposal was subject to completion of a four-week period of exclusive due diligence to AGL’s satisfaction; unanimous recommendation from the Vocus Board; and entry into a mutually acceptable scheme implementation agreement.
"Any scheme implementation agreement would also be subject to a number of further conditions, including shareholder, court and regulatory approvals," the announcement said.
AGL said in a statement: "AGL’s interest in Vocus is consistent with AGL’s strategy to meet the needs of increasingly connected customers as energy and data value streams converge and the traditional energy sector transforms – and aligned with AGL’s capabilities in integrating and managing complex assets and customer portfolios.
"AGL believes acquiring Vocus may be an optimal way of executing this strategy, creating material shareholder value and driving customer loyalty while providing access to a market-leading integrated broadband fibre asset base and creating considerable additional options for long-term growth."
Vocus has appointed UBS as its financial adviser and Allens as its legal adviser.