Andrew Purgacz was drunk and lost in an unfamiliar city.

More specifically, the Adelaide tourist was 150 metres inside Sydney's Garden Island naval base, one of Australia's major defence facilities.

Mr Purgacz, 37, was found sleeping on a grass verge within the inner-city base at 11.02pm on December 23. He was arrested by Federal Police and charged with trespassing on prohibited Crown land.

According to a police fact sheet, he had spent 12 unaccompanied minutes inside the perimeter after he walked around a boom gate and past a number of no-entry signs.

"It is mind-boggling that security at Australia's number one naval base was so slack," Mr Purgacz's barrister, Peter Lavac, told the Downing Centre Local Court on Tuesday.

Most of Australia's naval assets are in the Garden Island Fleet Base East or Perth's Fleet Base West. Handwritten notes seized in counter-terrorism raids in December 2014 allegedly referred to the base as a possible attack target.

Mr Lavac said his client was of excellent character and had recently spent three months off alcohol as part of a health regimen.

Mr Purgacz had been drinking with his mother and sister on December 23, starting with a bottle of rosé at Opera Bar before they shifted to the Golden Sheaf Hotel in Double Bay.

Then the three went on to the bar Mrs Sippy in the same suburb for "further libations". None ate anything throughout the night, the court heard. All up Mr Purgacz drank about 10 alcoholic drinks, the police fact sheet said.

Mr Lavac said his client took a cab toward his accommodation at Surry Hills but became convinced the driver was going the wrong way.

Mr Purgacz then decided to walk home but ended up at Garden Island. He claimed not to have seen any no-entry signs.

"If he had seen any signs he would not have been capable of reading them," Mr Lavac said.

Magistrate Alison Viney said the offender should have known better at age 37.

"If you're going to drink yourself into oblivion we would prefer you stayed in Adelaide," she said.

"In some countries you would have found yourself shot had you wandered into a military base."

Magistrate Viney did not record a conviction against Mr Purgacz, who pleaded guilty at his first opportunity, but placed him on a $500 good behaviour bond.

Mr Purgacz told the court he was sorry.

"I just had a bad night," he said. "It wasn't on purpose."

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