Both Optus and Vodafone have struggled in recent years with network coverage compared to Telstra, although whether that's been network congestion or network coverage has been a bit contentious (to put it mildly). The two companies have just announced that they've signed a memorandum of understanding to share 3G and 4G towers, which could mean massively improved service — or massive congestion.
The initial release came via Optus, so naturally enough it talks up the Optus angle, stating that Optus customers will gain access to nearly 1,000 additional mobile sites, providing around 20 per cent expansion in existing coverage areas. This isn't just in 3G areas; Optus' release notes that it'll allow it to expand 4G coverage in Sydney, Melbourne, Brisbane, Adelaide, Perth, Geelong, Central Coast, Gold Coast and Canberra, as well as accelerating Optus' rollout of services by a claimed 12-18 months.
The agreement will run for five years, during which time Optus will be the exclusive roaming partner for Vodafone customers, presuming the ACCC actually lets this go forward. More details as we have them.