However the company said that its guidance for earnings before interest, taxes, depreciation and amortisation for FY19 was not affected.
In an update issued to the ASX on Tuesday, TPG said the expenditure on spectrum purchase had not been written off as expenditure in its balance sheet as the spectrum was not going to be used.
Even if its proposed merger with Vodafone Hutchison Australia went ahead, TPG said the spectrum it owned would be complementary to the VHA mobile network.
"Therefore, pursuant to an impairment review undertaken for its 1H19 accounts, the group will reduce the value of its spectrum licences by approximately $92 million, primarily reflecting the fact that, as the licences have finite lives, their value necessarily diminishes over time," the update said.
The proposed TPG-VHA merger was announced in August 2018, with the scrip deal creating a company with an enterprise value of about $15 billion.
In December, the Australian Competition and Consumer Commission expressed some reservations about the merger, saying it would lessen competition in the mobile sector. The watchdog said it would give a final determination on the deal on 28 March.
In January, TPG said it was cancelling its plans for a mobile network. The $600 million network was announced in 2017. The company said the government ban on the use of Huawei equipment in 5G networks meant that it could not go ahead with its plans.
TPG said in Tuesday's update: "In these circumstances, an impairment review of these mobile network assets, undertaken for the Group’s 1H19 accounts, has given rise to a write-down of the mobile network capital expenditure incurred to-date of approximately $76 million."
TPG will announce its 1H19 results on 19 March.