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v3.25.2

Changes in the valuation allowance are as follows: (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Jun. 30, 2025 | Jun. 30, 2024
Income Tax Disclosure [Abstract] |  |
Valuation allowance, beginning of year | $ (28,181) | $ (25,463)
Current year additions | (3,978) | (2,718)
Valuation allowance, end of year | $ (32,159) | $ (28,181)

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- Definition The element represents deferred tax assets valuation allowance additions. + References No definition available. + Details
Name: | niobf_DeferredTaxAssetsValuationAllowanceAdditions
Namespace Prefix: | niobf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482685/740-10-50-2 + Details
Name: | us-gaap_DeferredTaxAssetsNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_IncomeTaxDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration