It's alleged the guns were being kept in a hidden underground vault at a property in High Wycombe, accessed via a trap door using a hydraulic arm.
The owner of the bunker, which also included a shooting range, is facing allegations he built it without council approval.
Premier Mark McGowan on Tuesday announced his government would ban "very high-powered" firearms capable of shooting over a long range and piercing armour plating.
More than 50 types of firearms and 19 calibres of ammunition will become illegal and a government buyback will be conducted.
Police Minister Paul Papalia said the bunker discovery showed the need to update the state's "outdated and weak" firearms laws.
"Unbelievably, the worst offence this individual has allegedly committed is failing to apply to the local council for approval to build the bunker and underground shooting range," he said.
"This person was a licensed firearms owner. Most of the weapons down there were licensed.
"Why he needed that many high-powered rifles in a secret underground shooting range is beyond me."
A total of 248 licensed firearms must be disposed of by the July 1 deadline and will be bought back at market value.
Owners must provide their firearm's licence, bank account details and proof of purchase when they surrender their guns to police stations.
Acting WA Police Commissioner Kylie Whiteley said some projectiles from the firearms were capable of penetrating body armour worn by police officers and hard surfaces of vehicles from up to two kilometres away.
"With specific types of rounds, a bullet from these firearms can also penetrate WA Police Force armoured vehicles," she said.
"This is why we have requested the state government ban this list of specific types of firearms and ammunition."
Mr McGowan said he had been advised there was no practical need for these firearms in WA.
"While they remain in our community, these weapons are vulnerable to falling into the wrong hands and the consequences could be devastating," he said.
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